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 classic 0.8mm 5-point star/Pentalobe. You can use these in either a 4 or 4S phone. Adafruit also have the Phillips version if you want to replace the screws with Phillips #000 flavor.
Comes in a pack of 2! You may also want to pick up a 0.8mm Pentalobe screwdriver if you don't have one already.