I re-raise this. No reason not to. If he is an unknown to you, than you are an unknown to him. Just because he raises it to $4 utg, doesn't mean he has QQ-AA. Probably has some broadway cards, or some pp if he is an aggressive player. If he plays back at you, you can easily fold, and this play will help establish you as a "loose" player, so you will probably get more action.

Edit: Just wrote what Fnord said and that is what I would do. He explained in a much briefer fashion