This is a copy, paste, and edit of f4780y's post geared toward 4.2.8 downgrade.
iPhone4 users are experiencing a recovery mode loop following a 4.2.8 downgrade which cannot be exited using the normal methods, such as TinyUmbrella's "Exit Recovery" option.
The following process has been proven to fix this problem, and has the added benefit of jailbreaking the device at the same time.
First you will need a copy of the latest (rc19) redsn0w jailbreak. This can be downloaded from here:
redsn0w_win_0.9.6rc19.zip for Windows -
download link
redsn0w_mac_0.9.6rc19.zip for Mac OS X -
download link
Next, you will need a copy of the 4.2.6 firmware. If you need to download it again, use our sticky here to source the right file -
Official Apple IOS Firmware Download Links for iPhone
Finally, you should download the latest TinyUmbrella, if you are not already using, it from here -
The Firmware Umbrella - TinyUmbrella
Windows7 and Vista users should set the compatibility properties for redsn0w.exe by right clicking on the EXE file, choosing "properties", switching to the Compatibility tab, and selecting "Windows XP (Service Pack 2)" and "Run this program as an administrator".
Go through the normal downgrade steps till you get the 1013 error. I used TU's TSS server for SHSH
Now launch redsn0w, select the IPSW file for 4.2.6, and follow the instructions. Make sure you set the "Install Cydia" item on the options screen.
The normal jailbreak process will now run on your iPhone, however THE PROCESS WILL HANG at the "Installing Activation Bundles" step. You should see the iPhone spinner in front of the pineapple. THIS IS OK.
You now need to reset the iPhone by holding power and home for 10 seconds.
The phone will now reboot and you will STILL be in recovery mode. DON'T PANIC!
Now, you can launch TinyUmbrella, click your iPhone in the list of devices, and press "Exit Recovery".
It WILL work this time. Your phone will reboot into an untethered jailbroken state.
Now you should have a fully functional downgraded and jailbroken iPhone which is
no longer stuck in a recovery mode loop.
I am an iphone noob so I only have this one experience with downgrading. I followed directions and maybe just got lucky. Hope this helps you guys.