Regarding the original question, if it were not for the different size sims, yes you could put your 3G sim card into the iphone 4s and it would work. However, I'm not sure you would get the higher data speeds offered by HSPA+. For that, I believe you do need a sim that is compatible.
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Re: Question: can you put a 3G sim card into a 4G iphone?
I don't think the SIM has anything to do with whether you get 3G vs HSPA+ speeds. That is based on the hardware in the phone and maybe your data plan also. The SIM just contains account info. I'm sure there are people using SIMs from their 4 in their 4S and they are working fine.