The Schubert Fantasie in F minor is maybe the greatest piece in the four-hand literature, IMHO. If you play nothing else, play this piece!
There are five fun Mozart sonatas for four-hands. Lots of Brahms: Liebeslieder and Neue Liebeslieder Waltzer (which are wonderful with or without the vocal quartet), the Op. 39 waltzes, and the Hungarian Dances. Dvorak's Slavonic Dances. Rachmaninoff's Op. 11 contains six fine pieces with the usual technical challenges this composer presents.
Then there are the French: Faure's "Dolly Suite," Debussy's "Petite Suite" and "6 Epigraphes Antiques" (originally 2 pianos, I believe, but there's a 4-hand version), and Ravel's "Mother Goose Suite" -- all great fun.
If you're in Christmas mode, there's Lizst's "Weinachtsbaum" (Christmas Tree), in his own transcription for four hands.
Barber's "Souvenirs" is also worth a look -- original four-hand pieces; I like it a lot.
And, of course, there are four-hand transcription of all of Beethoven's symphonies that are pretty incredible. Lots of other chamber and orchestral music exists in four-hand versions, many by the composers themselves.
You'll have so much fun....
Topic: Piano Duets