Dear Pei HUng,
I'm too a victim on my 4S, my unit just exactly 2 years old, it do not had warrenty any more, and for this problem I don' think is the ios 7 issue, is the 4S hardware issue.
I already fix the problem, but you must think I am a nut head that too free to wrote you this email, anyhow you make you decision to do it or not to.
I had been searching up and down throught the web side on this issue when my 4S wifi suddently grey off with no reason, a smart phone with no wifi for me not so smart phone and it consider stupid phone.
Apple do not actually had the solution and only wish we buy another new unit, infect I did, I brough a 5S and only I fix the phone. I had visit Apple store here and there in my town and cities, no solution, just buy new phone, or some event wanted 100 buck to fix it.
I had no other choice but to get the 5S ( S is stand for same old **** ) the phone is nothing new, just thinner and lighter, with ony few new stuff like smart finger print button and slowmo video that we usually do not event use it.
I had a mac book pro, ipad 2 (Son), airport time capsule, and another 4S (Wife), all this unit is interconnected specially face time, so I cannot switch to other brand.
If you are not like my situation, I recomment Samsung Note 3 is the best if you compare them with price to get the 5S.
Lets cut all the grandmaother story, and how I fix the phone?
I got this carzy idea on the You Tube, my wife mad at me and told me not to follow because the phone still usable for calling, and without the wifi as it is our main issue here.
After I got the new 5S, I start to follow this crasy thing I ever done, please refer to the link for your reference how to fix it : http://youtu.be/4mVkiU2jNSE
I do it with carefully, if you see the sreen turn black, blow the other end, it will show the unit is over head as seen in the you tube, then stop the blowing (careful the phone is very hot). Put it immediately in the freezer for several hours.
When you took it out from your freezer, leave it on you table to get some room temperature back as I am in tropical climate, and it get vaporate pretty fast, but make sure the phone is still ice cold, then turn it on, I am sure you wil get this issue fix just like me, sound crasy, but it works.
Once you had do this, please circulate this words around to help other to get their wifi back as Apple without Steve Job is another blood sucking company that do not care anymore for customer but our money.
Good Luck, and have a good 2014 ahead.