aha!
this piece is much harder then it sounds, not for the pure technique, but getting the voicing, and phrasing, ESPECIALLY on these octaves, to sound good.
definitely use 4 and 5, because for each group of 3, you have to connect the first two.
Practice with the outer voices alone, trying to connect the 4's and 5's of both hands.
There are more complexities to this passage then one thinks.
1. You have to play them a particular way: stress on the first of the triplet, connect on the 2nd note, and do a small bounce on the third, so it sounds like Strong weak weakandstacatto, strong-weak-weakandstaccato etc etc etc
2. You have to do a crescendo and descrescendo, the octaves are supposed to come out of no where, like something building up. Start incredibly soft, practice SO closely to the keys and don't lift until that third staccato note, and just a bit.
3. You have to pedal every 3 of them.
These 3 steps are difficult to combine, but when done, it sounds excellent.
Literally, spend time and write this out. First write the Tenuto on the first of each 3, the slur from each first note to second, and then the staccato on each 3rd note of the triplet. THEN, write the crescendo and descrescendo in. THEN write in the pedalings, and you can even choose to pedal every 3 notes that are the SAME, but that would be hard as hell.
After writing these in, work on the first component first (the motions) without anything else, and keep your hands close, play quitely with barely any movement, but connecting and lifting for the striking of the 3rd note in each triplet.
Then, work on adding the 2nd component, then the 3rd.
Definitely practice without pedal at first (hence the 1st and 2nd components being first)
trust me, it'll help a billion. My teacher Tatyana Stepanova (won Russian 81' international) totally enlightened me with the explanation, because I hated the way i played them, and didn't know why.
Topic: Chopin nocturne 48/1. Very fast octaves !