It is SIM unlocked. You should be able to get HSPA on AT&T. I don't believe you will get LTE. You might have to play with your APN to get SMS/MMS to work.
The Verizon s4 supports band 4 lte, which is att secondary lte band. Note that it was locked at release, not sure about now. Verizon would have to issue an update to unlock it. Also, att band 4 is very limited, but if you happen to live in a city that supports it, then you are in luck
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