I had been wanting to jailbreak my iPhone 4 for quite awhile but since I had upgraded to Apple iOS release 4.2.1 there was not an untethered jailbreak available at the time and since I typically turn my iPhone completely off I didn’t want to deal with the untethered jailbreak. So needless to say when GreenPois0n rc5 was released I was pretty excited. GreenPois0n rc5 made it extremely easy to jailbreak iPhone’s running iOS 4.2.1. After running it for around a month now though I noticed that I really don’t install enough applications outside of what is available via the iTunes App Store to warrant the jailbreak and especially because I have personally noticed that with my iPhone jailbroken it had slowed down a bit. Not only had it slowed down but my battery appeared to be draining much quicker which I assume was from more processes running. I made the decision a couple days ago to roll back to the stock iPhone 4.2.1 firmware to see if my phone started running more efficiently. Below are instructions for rolling back the GreenPois0n firmware on an iPhone 4 to the Apple iOS 4.2.1 firmware release.