I don't think anyone expects a 4 yo to join in on the adult conversation ... therefore, I think it's fine to bring the gameboy (and/or some other things). I'd rather have my child (and, in turn, my friends children) occupied than whining every 10 mins. that they're bored and want to go home.
Since the adults will be mainly conversing, why not bring a video or dvd along? Maybe rent something new to keep your child far more occupied?
Have fun!