I want to have a foursome?

Well either you can suggest it to him and risk having him think strangely of you. Or you can find it somewhere else behind his back and do it just one time, to get it out of your system.
Either way there is a bit of a risk. Good luck to you.

Yes, change it to three girls, plus you and him and I don't think he'll object. :)

This is something you might have trouble getting him to do or understand. If the communication between you two is good, bring it up to him. If not then you might have to hold off on the satisfaction of accomplishing your fantasy, unless, you want to cheat on him. Maybe you guys can both involve yourselves in something other than that ,at the moment, and try something a bit more understanding and acceptable to both. You can go to a swingers club and have sex in front of others and if things flow. Maybe you will get yours and he gets his. I say, talk about the things you want to do and take it from there. If he is mature enough, he will understand your needs and fantasies if not, then both of you can come up with what you are both willing to do.