mathews gensis cause he is going to be growing and the bow grows with you. once he starts to get to big for that you can start to look then, but get a genesis now
Not at four. My daughter is nearly the biggest 4 y.o. around and she can't even hold up a Genesis, much less pull it back.
Stacey Archery makes the smallest compound I know and even that was a little on the big side for her.
I ordered a Lil' Coyote long bow from Kustom King Archery with a 10# pull at 18". It's her Christmas present, but I let her 5 y.o. cousin (same size kid) shoot it and she shot it great.