Hi, I have 4s and had the same problem of grayed wi-fi since updated to IOS 7. I kept on finding solutions, read every single mail on this forum and tried every trick to solve this issue. When I first time cooled down my phone in freezer and exposed to hot hair dryer after that, wi-fi was turned on and it wept working for one day untill the battery was down. Then it never turned on, no matter whatever I tried. Some times I could see address of wi-fi in settings but sometimes not.
The part of the world I live in has no customer support centre of APPLE but I do have technicians who can do repairs of all kinds. They suggested me to replace the wi-fi chip but there was a chance of losing the phone. I was facing a slowed down phone with no use other than for making and listening calls. I was very frustrated and decided to take a chance of getting it repaired.
When the technician opened the set, I asked him to blow some hot air on the wi-fi chip. He separated the whole motherboard from the set and exposed wi-fi chip area to very hot blower for 1-2 seconds for 3-4 times and let it cool naturally. The blower was so hot that it could burn a cotton bud in 2 seconds.
He assembled the set again and when turned on, every thing was fine. Wi-fi and blue tooth were working fine and I also noticed a quiker response of apps especially of SETTINGS and PHONE.
I wish this repair may help me and other affected 4s users. I think we should not fall in temptation of new updates and be happy whatever we have. But if we want to update then we should first do it in iTunes on our computers and then through it on our phones. It will not overheat our phones. I don't agree to the statement that APPLE claimed that 4s had a hardware issue, exposed by iOS7. I think they should have suggested to the customers to download updates first on computers and then on phones plus it is very disappointing that they still have not noticed this problem and if noticed then haven't issued any suggestions.
APPLE we love your products therefore you should care your loyal customers.