Another one of those achievements where you leave your computer on while you sleep or work or...play the game.
Simply Load into a Single-Player or Multi-Player game, preferably Single-Player, and be in the game for 4 hours. The game cannot be paused either.
You can also get the "Marathon" achievement by being in the game for 4 more hours (8 hours total), so if you want to, you can get both of those done.