How to downgrade Verizon iPhone4 from 4.2.9/4.2.10 to 4.2.8 [Tutorial]
Let me begin this tutorial by saying that you MUST have your 4.2.8 SHSH key saved in order to downgrade from 4.2.9 or 4.2.10. This tutorial will not work if you do not have your SHSH key. If you do not know what an SHSH key is then you most likely do not have it saved. You can check out our What is an SHSH file post.
Step 2: Now that you have created your custom firmware for downgrading, we must trick iTunes into thinking that the firmware is being signed using your SHSH key that you have saved either locally or on Saurik's server using Cydia.
Step 3: Download the latest version of TinyUmbrella.
Step 4: Open up TinyUmbrella and click "Start TSS Server" (iTunes should be closed, if it isn't closed TU might close it automatically when you click start but shut it down before starting the TSS server just in case.
Step 5: Put your device into Pwned-DFU mode using iREB(Built into Sn0wbreeze on Win) or PwnageTool(for OSX) and then open iTunes and hold down shift(win)/option(mac) and hit restore. This will bring up a dialog box. In the box select your CUSTOM firmware that you created in Step 1.
Step 6: iTunes should now restore your device with no problems. Once your device is restored and reboots you will be downgraded to 4.2.8 and you will have Cydia pre-installed on your springboard.
If you have any questions or comments, please feel free to post in the comment section below.