If you haven't done the update of ios7 on your iphone4 yet, I strongly suggest that you keep it that way and NOT update it. If you did, unfortunately it's not possible to revert back. If your data issue is intermittent and happens everywhere, than it's most probably a hardware issue. Either your sim needs to be exchanged, or your phone's software is playing tricks on you. Try doing Settings>General>Reset>Reset Network Settings first (it won't delete anything except your saved wifi passwords). If that doesn't help, try a phone restore or exchanging your sim at a store.