Q: My 4s loses wifi and will not let me join back in.

I would follow this article:

iOS: Troubleshooting Wi-Fi networks and connections

Hope this helps!!

If nothing in that article is helping I would recommend you try connecting to a different wi-fi network..

This will help you isolate the issue further.

(Also this is random, but if your phone has a case on it, try taking it off)

If your phone still isn't able to connect to ANY network, you may try a backup ( http://support.apple.com/kb/HT1766 ) & restore ( http://support.apple.com/kb/HT1414 )

Set it up as new after the restore, if it's still not connecting it's something with the hardware of the phone that would need service.

Goodluck!!

=/ I'm sorry you're having so many issues with your poor iPhone...

If it's giving you a specific error when trying to make the backup try some of these articles :

iTunes :

http://support.apple.com/kb/TS2529

http://support.apple.com/kb/TS3683

iCloud:

http://support.apple.com/kb/TS4576

http://support.apple.com/kb/TS3992

I hope you get that iPhone working or you at least have AppleCare!!

Goodluck!!! 