Is it possible to use an AT&T gsm iPhone 4 with any other carrier or pay-per-use
Once the AT&amp;T iPhone 4 is unlocked, you can get a no-contract SIM from Straight Talk (which is sold at Walmart) or from T-Mobile. 

With Straight Talk, you're pretty much forced to buy a voice/texting/data plan for a minimum of about $50/month -- they won't let you use a voice/texting-only plan into a smartphone chip. Data is capped after 2GB.