First you need to figure out if you r PS3 can be downgraded (minverchck in my sig) then you need to get a hardware flasher and FSM dongle to downgrade to OFW 3.55 IF minvercheck says you can.
If not you are SOL.
and yes you need to be on 3.55 to install CFW of any type. You cant install 4.21 CFW over 4.21 OFW. It wont work, an exploit would be required to do so.
No freaking way