There is already a Sokoban implementation at pouët, by Rebels, in 1k:
http://www.pouet.net/prod.php?which=31727

Why a 4k then?
I wanted the full 50-level set and decent graphics based
on the original game. It's not often that you see a 4k game
with 50 levels ;)

I ran out of space to implement auto-save though...

Keys:

ARROW KEYS - move
N - next level
P - previous level
R - restart

dwim