How to Downgrade iOS 4.2.10 to iOS 4.2.8 on CDMA iPhone 4?

Hello Aaron, the only solution to downgrade iOS 4.2.10 (iOS 4.3.5 for GSM iPhone) back to iOS 4.2.8 (iOS 4.3.3) is via TinyUmbrella. But, of course, this process requires you to have a saved iOS 4.2.8 SHSH Blobs, which I don’t think you have. If that’s the case, then you are stuck on iOS 4.2.10 till Apple releases a new version of iPhone firmware probably coming this fall – the iOS 5.

Regarding the iOS 4.2.9 or iOS 4.2.10 (iOS 4.3.4 and iOS 4.3.5) untethered jailbreak, we don’t think that an untethered jailbreak is gonna come out anytime soon as the iPhone Dev-Team is focusing their attention on the upcoming iOS 5 firmware. If you aren’t aware, Apple released these firmwares to permanently patch the untethered jailbreak exploit used by the Dev-Team on their jailbreak tools as well as to address the security exploit which was utilized by Comex to jailbreak the iPad 2 using the new JailbreakMe 3.0 tool.

Anyway, if you are looking for a jailbreak solution for your device, then you can use Redsn0w 0.9.8b4. However, as previously mentioned, this jailbreak offers tethered method only. Meaning you have to connect (tether) your iOS device in to your Mac or PC and boot into a tethered jailbroken state every time you power off or restart your device. You can follow this tutorial on how to tethered jailbreak iOS 4.2.10 on iPhone 4 using Redsn0w 0.9.8b4.