Re: iphone 4S wifi button grey out after installing update
Hard restart (hold the home button and sleep button) can usually sort this out. If you still have trouble, reset your network settings or fully restore the phone, as mentioned above.
This WiFi issue is pretty common with 4S that were made for a few specific carriers. At some point, the WiFi greys out permanently because your chip has come loose, so the only fix is a refloat your WiFi chip. It's not an expensive repair, just make sure the shop has a good policy around this type of repair (e.g. refund if unsuccessful, 4 refloats after your initial payment if the fix doesn't hold, etc) because it's a tricky thing to make stick.