you can get a set of precision screw drivers from a hardware store for about $5 if you get them from a RC shop or get some "special" iphone tool you will be paying more im sure
I have purchased the one on ifixit for the mac air that claims to work for the IP4. Let me be the first to say that it is not the correct one. They refunded my money as soon as I let them know. This is a very small screwdriver that is not being manufactured by anyone yet. If anyone finds where to get it , please let me know. It is smaller than the t3 ones.. I also got some on ebay and they also were too large.
It is a philips 00, I have changed all 6 iPhone 4's backs in our family with no problem...
Pentalobe-Screwdriver- size TS1
or Philloip - Screwdriver- size 00
Would be cover all generation of Iphone.
You can also look in the ebay or amazon to get one.
soungz is correct. Make sure you take a look of what type screws you have before you stick something in there or else you risk stripping the heads and they'll be stuck in there forever!
Its definitely not a philips screw head, you just need to look at it to see that. Its a five pointed shape which is a TORX, which is used by apple in nearly everything they make. Size would be a 5 or a 6 I would guess as I haven't got my TORXs at hand.
A philips screwdriver wouldn't work, you could try a flat headed precision screwdriver if you couldn't be bothered to get the correct one, however you might find it hard to get the right size.