What golf club should I buy?

If I had to play with the four clubs you listed and I could only add one more I'd go with a hybrid club, probably something around 21 degrees of loft. That's a pretty versitile club that you could hit from tee, fairway, or rough pretty well and should give you a club to use on longer par 3's. If you prefer the look of a wood over a hybrid, I'd steer away from the 3 wood and go with a 5 or 7 wood; they're easier to hit, especially from a less than ideal lie, and more likely to be used on par 3's where you're likely to find that you have no way to reach with your 7 iron, but the driver is just way too long or totally the wrong club for the hole.