iPhone 4 because 3G is an advanced cellular network. If it was named IPhone 4G people will mistake it as a 4G phone, but it's really its a 3G phone. That's why it's called iPhone 4
No, and yes, sadly. Dumb AT&T are keeping it at 3G, unlike Verizon and T-Mobile. If you really want 4G on your iPhone, then get the Verizon iPhone 4. Much better.
It is the 4th model of iPhone. But iPhone generations do not go from one up: 1. iPhone 2. iPhone 3G (because of 3G connection) 3. iPhone 3GS 4. iPhone 4. 5. iPhone 4S