If you drink four beers at 4 percent how much of that is in your system?

Depends on what you've eaten, how quickly you consume them, and how much you weigh. If you were a male weighing 180, I would guess your BAC would be around .06. Here's a helpful chart:

http://encarta.msn.com/media_461544063/percent_of_alcohol_in_bloodstream.html