Hi,
I too have an iPhone 4S which lost it's WiFi about 6 weeks ago, and I tried everything to get it back, short of exchanging the phone itself. I was hoping the upgrade to iOS7 would solve the problem but it didn't.
Tonight I read a post which suggested that using the inbuilt LED for an extended period might help. How this could possibly work I have no idea, but I'd tried everything else so I gave it a shot and to my utter amazement my WiFi is back working again!
This is what I did;
I have an appcalled "Light" which simply turns the camera LED on so it can be used like a flashlight. I switched the LED on and left it on continuosly for about 10 minutes.
At the end of 10 minutes I went into my settings and checked the WiFi option, which had been greyed out, and lo and behold the option to turn WiFi on was now available. I switched WiFi on and my phone immediately picked up my home network, something it has not been able to do for weeks.
It has now been about 2 hours since my WiFi came back on, and it's still working. Up until now I was only able to get WiFi back for about 10 minutes max.
While it's still early days, I'm very excited and hopeful that this solution may have overcome the issue, at least in my phone.
I will post again to advise the status of my phone after 24 hours. If this has solved the problem, it proves conclusively that the WiFi issue is NOT hardware related, but some kind of software glitch, possibly caused by the previous iOS version.
Best of luck!