Oh. Boy.
I have had a LOT of games that I could say that defined my childhood, but ONLY four games? That, to me, is a toughie. However, I know four that i remember very well.
-Sonic And Knuckles (The FIRST-EVER video game i played on Genesis.)
-Super Smash Bros. (N64)
-Kirby 64:The Crystal Shards (N64)
-Perfect Dark (N64)
And that's it.