So I had an idea that I'd like to go back and re-learn the piece that I played for my first ABF recital, to see how my playing has changed over the past 4 years. Is that "cheating", to play the same piece again for a later recital?
Not at all. The contrary actually since you will be able to show your progress and improvement of the piece.
We should at some point have a "re-cital" in which you can replay previously performed pieces that you just want another crack at. Of course, you can perform any piece you want in the ABF recitals, even if it is the same piece you performed previously. It's all good.
As for me, I don't have a clue what I'll be playing next. It doesn't matter either because I can be practicing a piece for a while only to hear a nicer piece come by and then I'll want to play that. So I do. I purposely do this as well to expose myself to as many different types of music as I can as well as try to figure out different sounds/chords etc.