These are perfect replacements for the little screws on the bottom of iPhone 4/4S's. Adafruit have them in two 'flavors' - this flavor is the slightly easier-to-work with Phillips #000. You can use these in either a 4 or 4S phone. Adafruit also have the 5-point version if you want to replace the screws with Pentalobe flavor.
Comes in a pack of 2! You may also want to pick up a Phillips #000 screwdriver if you don't have one already.