Recently faced an issue when trying to update an old iPad 2 from its original 4.3 OS. Back in those days even OTAs were not supported and iTunes was the only way to update. Since Apple has switched off validation for iOS versions older than 9, the iPad was going into an “Error 9” unusable state and there seems to be a lot of frustrated users out there who seem to be trying to update to iOS 9 from 4.X.X and failing to do so.
Some Googling turned up a suggestion to update to the beta version of iOS 9, and voila it worked. Unfortunately, you most likely need a developer account to actually download the iOS 9.3 beta file as the public beta is unlikely to work with iOS 4.3. Nonetheless, if you manage to get hold of the iOS 9.X beta version then definitely give it a shot.