Do you have any suggestions on Chopin pieces that are at the same level as the 1st ballade? How about the Polonaise-fantasie or the Fantasie Op 49? Or are there any Beethoven sonatas that are about the same difficulty as the 1st ballade?
Nah, both the fantasie op. 49 and the polonaise-fantasie op. 61 are about as hard as that 4th ballade, if not more.
The best suggestion would be to tackle the 3rd or 2nd ballades - the 3rd being easier than the first, the 2nd harder. Also, though the fantasie-impromptu op. 66 is easier than the first ballade, it builds a lot of technique, so it's a good choice.
You can also try out a scherzo - probably the second or third. If you like the first you can try it too, but stay AWAY from the fourth.
I have some other suggestions, but I think they're not as good as the ones suggested above:
Polonaies - Eb minor, Bb minor, D minor.
Sonata 1 in C minor, mvt 1.
You can play some etudes, some of the can come in handy (Technique-wise) later while studying the 4th ballade.
Preludes - 24 in D minor, 22 in G minor, 19 in E flat major.
Those are some of the things that come in mind, but I'm not really sure about them. (It really depends on how you play.)