Martinu wrote two piano quintets and a piano quartet, all three of which are excellent. I would also recommend Faure's piano quartets and Milhaud's later piano quartet, op. 417 (1966). Being a huge fan of French and Parisian musics from 1875-1950 (later romanticism through neo-classicalism) I listen to absolutely tons of chamber music.
Faure is of course excellent, and just as good (and better in some ways) are the works of his pupil, the Romanian Enescu. The latter is excruciatingly difficult thought.
And Fauré.