Of the small output for the four-hands piano by Beethoven, the most important is the two-movement Sonata in D, Op. 6, published in 1797. It's generally considered as having been conceived as a teaching piece, given its relatively modest technical challenges, and the absence of documentation for any public performance.
Nevertheless it presents some interesting points: for example, the recapitulation is ornamented in both movements, breaking away from the Mozart-esque tradition of a straight reexposition.
