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4G SIM card on iPhone??

If your SIM slot is unlocked, yes. You will still only get 3G services however.
 
If you have a Verizon iPhone 4s, it doesn't need to be unlocked.
 
The iPhone 4S uses HSPA+ on AT&T, whereas the other iPhones don't. Therefore if you don't have a newer sim card on AT&T and haven't changed to a "4S" data plan then you won't get the HSPA+ (4gdata speeds).

Iphone 4S= 14.4mb per second
Iphone4= 7.2mb per second
 
I know HSPA+ is not really 4g, but you still can't hit those speeds without an updated sim card. They shouldn't have called it 4g to begin with. It's 3G enhanced.
 
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