AT&T sim from 4 to 4s not working

[aphexacid]

I bought my iPhone 5 and want to give my fiancé my 4s. She has an iPhone 4. Both on AT&T. I took her sim, put it in the 4s and it will not activate. It keeps saying I need to insert a compatible sim or go get a replacement from my carrier.

Wtf?

I tried a dfu restore too. Nothing.

 

[kappaknight]

Did you connect it to her laptop and restore it from iTunes?

 

[kellen]

I did the same, worked fine for me.

 

[aphexacid]

I can't even restore in itunes. It gives me the same error message the phone does. Incompatible sim etc etc.

What the heck do i do??