IDG Answers is a community of experts who are passionate about technology. Ask a question or answer one below.
Check this guide, it's really helpful:
P.S. I fixed the link.
Depends on the iPhone 4. Apple switched over to pentalobe screws during iPhone 4 production. Those are those little 5 point (ahem....penta) screws. You should be able to pick a pentalobe screwdriver pretty easily. Last time I was at Sears they even had them there. Other than that, if you have a few small screwdrivers, a case opening tool or an X-Acto knife, and a "screen removal tool" aka suction cup, you should be sufficiently equipped for any recent iPhone.