I just installed 3-point harnesses in my hatch and removed my door-mounted belts. Installation was really easy, and relatively cheap. Unless you are planning on doing a bunch of autocross and the like, you should just stick with 3-point harnesses. For one, they're a little easier to install. The ones I have are retractable so that you can still get at your glovebox or whatever without having to take off the belts, but I don't think they make 4-pointers with retractors. Anyways, here's my thread about my installation with pictures:
http://www.automotiveforums.com/vbulletin/t97588.html