"Easy" is a rather broad characterization, but nevertheless:
There seem to be 4 Duettos, BWV 802 - 805 by J. S. Bach, apparently playable on the piano.
I know of no Handel originals for 4-hands.
Haydn wrote "Il Maestro e Lo Scolare", a set of fairly simple variations.
There are a few fragments by Mozart which could be considered somewhat easy; the several 4-hand sonatas are definitely more complex.
Beethoven contributed a sonata in D, several marches, at least two variation sets.
Schubert's multitudinous contributions are legendary, although many or most of them are well beyond "easy".
Schumann wrote several items: 12 Pieces for Large and Small Children, op. 85, are the easiest.
And then there is Brahms, who composed a set of 4-hand variations, the Hungarian Dances, and a 4-hand version of the Op. 39 Waltzes.
Many of these items are already in the IMSLP library. See the link below:
http://imslp.org/wiki/List_of_Compositi ... Four-Hands
You might also wish to explore "44 Original Piano Duets", published by Presser for $18.95. Catalogue number is 410-41166.