Looking over this piece and I'm stumped trying to interpret the 4s in bar 29 and the 3s in bar 30. Also bar 30 is perplexing because there are 6 quarter notes. The piece is in 6/8 time. I don't think these are fingering suggestions. Bar 30 is completely confusing.
The 4s and 3s both indicate tuplets, as indicated in other answers. However, the quarter-note triplets especially, being equivalent to just writing eighth notes, beg the question: why did Alkan chose this confusing notation?
The reason is to clearly communicate the slowing of the music as this portion reaches its conclusion. To see this, consider also the preceding measure, 28. Here is a transcription of measures 28–30, just the right hand, which is the key element.
X: 1 T: Prelude in Ab minor T: measures 28–30, right hand C: Alkan M: 6/8 K: Ab minor =g'/2b'/2b/2e'/2b/2e'/2 b/2e'/2b/2e'/2b/2e'/2 | (4be'be' (4be'be' | (3(b2e'2) (b2(3e'2) (3(b2e'2) ||
Considering 6/8 as having two beats per measure, the music moves from 6 notes per beat to 4 notes per beat to 3 notes per beat. At the same time, Alkan has indicated molto rallentando. The quarter-notes, then, best communicate the musical intent, though confusing at first glance.
Were Alkan to have extended the phrase two bars further, they presumably would have looked like this:
X: 1 T: Prelude in Ab minor T: imaginary measures 30a and 30b C: Alkan M: 6/8 K: Ab minor (2b4e'4 (2b4e'4 | (2b8e'8 |
...expressing 2 and 1 note per beat, respectively.
The piece is in 6/8 time.
The 4's in bar 29 indicate quadruplets : four 8th notes in the duration of three. The '4' is notated in the wrong place, it should be on the beam side.
The 3's in bar 30 seem to be wrongly notated, they should be on the stem side and have brackets. They are presumably intended to denote three quarter notes in the duration of three eighth notes. The composer could (and should) have notated the bar as six eighth notes.
I agree with Tom Serb that it's not a great notation, but I think they must be tuple markings rather than fingerings, if the time signature is 6/8. As the OP points out, that's too many 8th notes, so I think it means "take the main beat unit, the dotted quarter, and subdivide it into 4 instead of 3."
As you say, bar 30 is confusing. My best guess is that it's hemiola, and has in fact fused two bars into one. Here the quarters are worth two (normal) 8th notes (not the 4-tuple 8ths of m. 29); thus the first Bb-Eb-Bb is one 6/8 measure and Eb-Bb-Eb is another, but they are notated as one measure to emphasize their grouping into sets of two quarter notes, with the second Bb "belonging" musically with the next Eb even though it would be in a different 6/8 measure.
Meaning that it could have been notated:
X: 1 K:Cb L:8 M:6/8 (b2 e'2)(b2 | e'2)(b2 e'2)
All of which sounds really confusing, but translates simply to a progressive slowing; with the molto rallentando added, I'd be inclined as a performer to "ease" my way from ordinary compound time 6/8 to the "square" 4-tuple 8th notes to the quarter notes—the point is it keeps relaxing and getting bigger.