Adafruit Aluminum iPhone 4S 0.8mm 5-Point Star/Pentalobe Screwdriver [ADA888]
We tried half a dozen different 5-point star/"pentalobe" screwdriver tools to find the one we liked the most The body is made from a single piece of aluminum and has nice knurled grips. The end-cap rotates so you can keep the body steady while rotating. The tip is nicely machined and is magnetic so it holds the miniscule screws in place while inserting/removing them.
Pentalobe 0.8mm screws are used in iPhone 4S (not iPhone 4). Check the bottom of your iPhone if you're not sure whether you have pentalobe or Phillips #000 screws.