Are you sure, it's a piece by Mozart?I know of a Sonata in D for 4 hands by Beethoven, which is op.6
Dear Sophia: The opus 6 by Mozart was published in Vienna, 1784, and it contains three sonatas, K. 330, 331 and 332. So, the piece you are asking for is the Sonata K. 330 (opus 6 n. 1). However, this is a solo piece. Two questions: are you sure that is Mozart the composer of the piece you want? And is the opus number correct? One last possibility is that you know this piece in some kind of arrangement. I know four-hand sets of the sonatas K. 283, 457 and 545. However, this is not serious: the great master have several works for piano duet, so why waste time with second hand stuff? Hope it helps! Best wishes!