To verify which screws that you have installed on your iPhone 4/iPhone 4S, refer to the image below. If the screws are normal philips screws, then all you will need is a #00 Philips Screw Driver. If you find that the screws are pentalobular, you will need this screw driver to open your iPhone 4/iPhone 4S.
Click image to enlarge:
Product SKU: p012294