Apple doesn’t sign iOS 4 any more, it won’t be possible to revert to a previous version. The only one signed by Apple and available for your device would be iOS 7.1.2.
The iphone 4 is now completely obsolete. Apple should never updated the iphone 4 to ios 7. ios 5 should have been the final update for the iphone 4 which it ran very good.ios 7 makes the iphone 4 completely unsable as a result of the old hardware.