Can I unlock a GSM iPhone 4 so that I can use a CDMA SIM card and make calls using the CDMA card?
Short answer is no... the GSM phones cannot work on the CDMA networks, even when changing SIM cards. The hardware is actually slightly different.
If you have a GSM iPhone, you'll only be able to unlock it and move it between cell companies using GSM.
This article gives some additional info on the topic...