What kind of SIM card does the iPhone 4 use?

The Apple iPhone 4 uses a Micro-SIM card.

The Micro-SIM (Subscriber Identity Module) contains the same chips as a Mini-SIM card, with less surrounding plastic. Micro-SIM cards measure only 15 x 12 x 0.76 mm. The Apple iPad Wi-Fi + 3G was the first Apple mobile device to feature the Micro-SIM.

Previous iPhone models used larger Mini-SIM cards.