Will Simple Mobile work on an IPhone 4S?

I'm about to buy an iphone off my friend. The phone is unlocked. Will I be able to use the 4g internet, and other things with no problem? He also says that I will have to go to tmobile to get it activated.

If the Iphone 4s is an ATT phone that has been unlocked.yes it will work on Simple Mobile, or any T-Mobile MVNO. BUT be aware the internet will be extremely slow it will only work on 2g or slower, because the protocol for the internet is not totally compatible with the T-Mobile Network.
I suggest if you want a prepaid use the NET10 ATT chip, $50 Unlimited Talk, Text & Data per Month prepaid… You do not need to unlock the phone and it will work at full 3g or 4g speed depending on the software in the phone. One caution the MMS may not work, but there's a jailbreak to bypass this issue.

Hope this helps