The fourth is obviously the hardest for many objectives reasons, tough some exceptions might find some other one harder for various reasons.
I don't disagree, but if you wish to contribute to this thread, it might behoove you to elaborate a bit - what, in fact, are some of these objective reasons...
F Major. It's the weakest of the four musically, I think, and requires sophisiticated interpretative skills to bring it off.
Why would Nr 2 be the weakest musically? It seems like a prejudice to me. You will often hear, Nr. 4 the "sublime", number 1 the "great", number 3, the "graceful", number 2 "the weak". Personally, I find 2 one of the most amazing pieces chopin ever wrote, an incredible drama of highest intensity, a gem perhaps only surpassed by number 4. In addtion, it is so well-written that it is probably the easiest to get across persuasively.
just in case you do not know i was talking about the Brahms four ballades op.10genius's i like how you automatically assume that i am talking about the Chopin ballades
The beginning will be boring in many hands because it's basically 1.5 page the same thing, it requires some insane musical skills to pull that off convincingly.