you can go back and forth at will. Just use the 3.55 downgrader he linked you to and whenever you want to go back to 4.21 just install that PS3UPDAT.PUP. If you make an /updater folder on dev_hdd0 and make subfolders named 01, 02, 03, etc, etc you can store your updates there and just pick the one you want to use from the debug update menu. You can name the pup files anything you want to identify them and they'll show up correctly
Fiery the angels fell. Deep thunder rolled around their shoulders... burning with the fires of Orc.