Um... well $4 billion buys quite a few games I suppose. That's not all I would spend it on though, I could probably buy every game ever made and still have like $2 billion left. I would most likely buy some random medium sized island, then research a way to grow mindless zombies, grow large amounts of said zombies, release them onto the island, and charge people to go there and slaughter zombies. With the money I get from visitors I could keep my zombie island open for quite some time, and probably have enough to live comfortably on as well. An obvious plus is that now I can live out my fantasy zombie apocalypse! Fun times!