Q: can't activate iphone 4s

There is no fix here, as your problem is usually a sign the the phone was hacked/jailbroken. If it was, then you cannot get help here for jailbroken phones. Your only option is  to get help elsewhere.

Possible causes:

1. Your antivirus is blocking access to gs.apple.com.

2. Your phone is jailbroken.

3. Your computer has been used at some time in the past to jaibreak some iOS device (not necessarily the phone with the problem), and its network database was corrupted by the hacking software that was used.

4. Apple's activation servers are down (very rare, and never for more than an hour or so).

For 1. try disabling your antivirus.

For 2. & 3. you need to go somewhere other than an apple forum for help. Although you can try deleting lines containing gs.apple.com from your "hosts" file on your computer.

Are you the original owner? I think you said you are, but just want to verify. Also, did you buy it from either an official carrier or directly from Apple, or from an independent shop? It sounds like it is locked to a carrier, and was hacked to unlock it. When you updated it this re-locked it to the original carrier.

Are you positive?  Where did you buy the phone? Is it pre-owned?

I would suggest using this article http://support.apple.com/kb/TS3424. If that doesn't work, then you may need to contact Apple (chat support is free) and they will have some more steps for you

Awesome! Glad that worked for you!