To restart your iPhone 4, simply press and hold a lock button for a couple of seconds. This simple step may help many times, especially with issues like this one is.
This simple solution worked for many, so make sure to try it yourself. All you need to do is change the current ringtone. Go to Settings, Sounds, Ringtone, and choose the new ringtone.
You know those strange things you do, and then the phone works just fine? This is one of them. Simply tap your phone with three fingers a couple of times.
In most cases, the reason why you are experiencing the bugs on your iPhone 4 may be because you did not upgrade your device. Once you do that, the bug may disappear, and most likely vibration will work just fine
This can help as well, but before you start, back up your files. Go to Settings, General, Reset and choose Erase all Content and Settings.