For the iPhone 4 on baseband 04.10.01, you have 2 options.
Option 1: IMEI unlock from a carrier or bought online on ApplenBerry (recommended and trusted) or eBay and such (at your own risk. Make sure you do your homework on the seller before buying)
Option 2: If your iPhone 4 is on iOS 5.1.1 or lower, and you don't plan on ever upgrading the iOS again until the Gevey comes out with a new iOS upgrade, then you can get the Gevey SIM for the iPhone 4. Once you upgrade your iOS, however, it won't work