The text is from S. Pauli to the Corinther, chapter 13. You may have everything, but if you don't have love, it's worthless. Without love we see in a mirror dark words, but with love we see directly. With love I will see it as he sees me. But now remains from faith, hope and love, love the most important. Love means in this context something like charity. If you have a look at the rest of the Corinther, it will become obvious that Brahms has selected carefully his metaphysical passage, although it is also a very famous one.
This song is written in 1895, a year before the other three songs. It is also different in character, both with respect text, with its focus on love instead of death, and with respect to music. But nevertheless it is seen by Brahms explicitly as the last part of a cycle, and has also with respect to its text some logic compared with the other songs. If you summarize the cycle, then the first song states that you have to be happy in you work, because you don't know what will happen after you life. The second song is very negative about current society, and concludes that it is better not to be born at all. The third contrasts the bitterness of death in the midst of a good life with the relief death may give for people with nothing to hope. And then, finally, the last song suggests that love is the most important thing in life.
The song starts in Es major, but the first chord is an As chord (IV), the As being the same tone as the highest tone in the chord, Gis, at the end of song 3. On the other hand, the transformation from E major to Es major is a transformation in another world by its halftone move.
The structure of the song is A (m. 1-47), consisting of three variations of the same theme in 4/4 measure, B (m. 48-75) consisting of two variations of another theme in 3/4 measure, and a Coda, consisting of a part build on the first theme (m. 76-82), and a part build on the second theme (m. 83-99).
The piano prelude already introduces two important motives in the song: The Es-F-G in the right hand and in the top voice of the left hand, and the octave jumps, as presented in the left hand. May be also the rhythm of 1/8-3/8 is a motive used during other parts of the song.
When the voice joins, the piano repeats the same two motives in a faster speed till m. 6, where in m. 6 the left hand does the motive of the right hand in countermotion. The voice starts with the octave jump, followed by the rising motive in double speed, and introduces another characteristic motive: along note, followed by a short one, as with the octave jump and also on "Menschen", "Engels" and "rede". After the octave jump of the voice there is a long line till the "Engelszungen" and back till the first Bes. This enormous line in combination with the major tonality, starting and ending the melody on the V (Bes), and the atmosphere of the main motives.
The second part of the sentence is a contrasts. If you don't have love (charity), then all those positive things are without value. The voice starts this motive an octave lower than it ended and without piano accompaniment, and then introduces a very very mild motive as a variation on the inverse of the first motive (D, followed with the inverse of the first motive in minor: F-Es-D). This is a very mild motive, symbolizing love, and continues and develops in the piano during the word "Liebe". The second half goes back to the real meaning of the first sentence; it's only a tinkling cymbal. The voice starts this part with a downward seventh jump, the counterpart of the nice octave jumps and the triad before. The downward movement of the voice in m. 10 is a diminished chord. The continuous repetitions of Bes octave jumps in the bass and the downward movement of chords in the right hand piano represent the uncomfortable feeling of doing something beautiful without love, but in general the optimistic atmosphere remains. Also in the second half of the sentence the voice goes form V to V, where the piano brings it back to the tonic Es at the start of m. 13, decreasing the speed of the "klingende Schelle".
And this is immediately the start of the start of the variation on the theme, that is presented in the tonic. The piano prelude is compressed in one measure instead of two, and in m. 14 the rhythm of the piano is half speed compared with the rhythm of the first two bass tones in fm. 2. The voice takes the first half of m. 3 much faster and the second half slower in m. 14, where m. 15 is a variation on the same measure. Then the voice takes material of m. 4 to vary it in m. 16, and integrates material of m. 14and 16 in m. 17. Then in m. 18 a variation on m. 5. The harmony in m. 15-18 follows the harmony in m. 3- 7, except for the replacement of Es7 by Bes7 on "Geheimnisse". The melody in the piano goes downwards instead of upwards in m. 5. In m. 19-20 the statement about "Glauben" includes already part of the second sentence motive, but in bes minor. The line goes in all directions, suggesting perhaps that faith without love is without direction. It motive is presented in full octaves, and end in octave jumps, and a downwards scale in bes minor, where the enormous jumps in the piano and voice may represent the "Bergen versetze" by "glauben". The piano echoes the "berge versetze" to end the first half of the sentence, where the last chord is a Bes chord without third. The leaving out of the third leaves it open if it is a major or minor chord. Perhaps this also has a symbolic meaning?
The second half of the sentence repeats more or less as a refrain the first exposure, except for having a Des instead of D in the voice in m. 24. Recognize the repetition of "wäre" in this sentence. This is required to get the low number of words on the same musical line, but also pays attention to one specific word.
The postlude of the second variation of the theme jumps immediately (skipping an introduction) in the third variation of the theme. The third variation has a much more complicated harmony then first two variations. The sequence in m. 31-2 intensifies the expression, as does the repetition in m. 33 of the downward movement, that is repeated in a variation in m. 35 again. In this same measure the fast unrest giving octave jumps introduced in m. 1 appear , with accents on the first note, symbolizing the "brennen". The harmony of this passage starting from the end of m. 33 is in f minor.
The second half of the third variation is much longer than the ones before, and a measure rest of the voice is before its start (as was also the case in the second variation). The word "Liebe" is extended over two measures, using the 3/8-1/8 motive, and having a very fast harmonic development f-bes-Es-As below it. The last part of the sentence is repeated after an piano interlude using material as presented in m. 33, 21 and 10-11 on two diminished chords. After a short pause in m. 43 the voice repeats part of the sentence, again with an accent on "wäre". The musical material includes both motives of the first half of the theme and the second half of the theme. For example, the Bes-As-G line in the bass of m. 43-4, and the 3/8-1/8 rhythm of the voice are all of the first part of the theme.
The last variation of the first theme is on its minor tonic in m. 46, but transforms in m. 47 towards a Ges7 chord, that is enharmonically the same as Fis, the V of B that is the start of the Adagio. The B can be seen as a heightened V of Es dur and is in this way related with the tonality before, but creates at the same time a completely different atmosphere, as is consistent with the content of this second part of the song. The text tells philosophically that we can only look into a dark mirror, where you learn only part of the truth.
The 3/4 measure makes the music more dreaming than the 4/4 before, as does the change in harmony. The piano follows the voice, so is much less independent than in the first part. The last note of the piano triplets are echoes of the voice, perhaps symbolizing the mirror effect in the text. The first notes of the triples are a simple but beautiful countermelody. The bass repeats the short-long motive in a slow fashion. The voice melody in m. 49-51 is a sequence, using the 3/8-1/8 rhythmic motive (even the whole rhythm of m. 3 excluding a normal length of the first beat) of the first theme. The piano intermezzo in m. 53 is an echo in the top notes of the voice in m. 52.
The second part of the adagio theme start with a seventh jump, related with the octave jump in the first theme. The voice of m. 57 is the same as 48, only a tone lower. And the triplet movements stop on each third beat, increasing the expressiveness of the music. The end of the piano triplet line flows into the third voice tone in m. 56-58, perhaps expressing the "von Angesicht zu Angesichte". So, in the first phrase the piano mirrors, while in the second phrase the piano anticipates the voice.
M. 60-72 are a variation on the theme presented in m. 48-60. The piano interlude in m. 60-61 repeats the starting motive and restart the continuous third triplets. Its rise towards an E suggests the happiness feelings of the text before. The E chord on the first beat of m. 61 is a very special one that interrupts the progress from B-Fis7-B (I-V7-I) with a IV chord. The melody line is a variation on m. 48-49, where the voice on m. 62-64 (and the echo in the piano in m. 65) is again a sequence based on these two measures, almost becoming a sequence of only variations on m. 48.
The second part of the theme goes to the mystical part of being known completely, and starts the same as in m. 54. But starting with the last beat of m. 68 the development changes. On this last beat the triplets starts to continue again, the harmony goes to a diminished chord in m. 69and the voice rises instead of remaining on the same level (but with the same motive as in m. 57). On "Ich" in m. 70 the voice has the only grace notes in the whole "Vier ernste Gesänge". The B is part of a diminished chord in the right hand piano, where the bass is a progress from a (diminished) cis chord towards Fis through Fis7 reaching the tonic B in m. 72. Below "Ich" the piano triplets become shorter with longer end tones, perhaps suggesting the reaching of knowledge even more than in the measures before.
Starting with m. 72 the motive of m. 48 is used to transform from the Adagio B major part towards the Es harmony of the first part of the song. Recognize that even in m. 74 and 75 the motive is repeated in the middle voice of the right hand piano, where the reprisal of the first theme is anticipated through the soprano and the octave jumps in the bass.
The part starting with m. 76 may be seen as a sort of coda, with m. 76 using material of the first part of the song, and the second part (starting with m. 83) focusing on the adagio part of the song. The first part starts with rather direct variation of m. 3 and 4, but then develops into a line that has only slight resemblance with m. 5 and 6. The chromatic chain in the middle voices (from Es in m. 78 to Bes in m. 81) increases tension, as do the rising fifths in the voice in m. 78-79 on "Glaube" and "Hoffnung" (the last on "Liebe" is delayed till m. 89). The word "Liebe" is accentuated by the high pitch of the voice (for a bass!), the long duration of the music on "Liebe" compared with the other words and the fast harmonic development by dominant relationships from g in the second half of m. 79 through C and F7 into Bes7 in m. 81. If you are looking back at the whole cycle, it seems obvious that Brahms didn't believe much in the relevance of faith and hope about life after death, so it seems obvious to me that the text has much more importance for Brahms than it has in the context of the Bible.
The ritenuto in m. 82 is the transformation to the second part of the coda. It is just a Bes7 chord with its solution into Es in m. 83, but the Fis as the last tone of the voice and the left hand piano gives extra pressure towards the start of the new theme.
In the Sostenuto the adagio theme is taken back, but integrated in the Es dur tonality. The triplets in the piano are doubled compared with the adagio, and the top line just follows the voice line, but a sixth plus octave higher, and at the end a third higher. In m. 85-86 the right hand piano has triplets where the voice sings two notes, giving an interesting rhythmic pattern.
Starting with m. 88 the voice takes elements m. 78 and further (related to
the first theme), where the left hand takes the triplets ending at the last
beat, taken from the second theme. In m. 91 the piano speeds up in an implicitly
2/4 measure, where the voice remains in the 3/4 measure. The word "grösseste"
has more or less the same atmosphere and accentuation as "Liebe" in m. 80, where
the voice line in m. 93-94 remembers vaguely both to m. 78 and further and the
start of the song. In the piano postlude the right hand piano has in m. 95-96 a
2/4 beat against the left hand 3/4 beat. The countermovement of left hand and
right hand in the Es chords, the arpeggio's in the two-beat in the right hand
and the ending on three repeated
Es chords, where the last chord is build of the natural tones on the bass line, as was the first chord in the song, gives a feeling of reconciliation.