The 4s I'm using currently used to have this issue when it belonged to the missus. Since restoring it, it's been fine. The phone is really starting to show its age though, which is a shame because it's by far the nicest one they've done to date in terms of build quality.
Wifi on iPhone 4S has just stopped working, looks like it can't find the wireless adapter.
Turning off/on doesn't fix it, hopefully apathy will and it'll start working again.