yes you have to be on 3.55 to upgrade to 4.21 and you can downgrade back to 3.55 again sound like a dex fw. Maybe someone can crack it soo we dont need the blue disk like the true blue days...
PS3-Slim 320g CECH-2503B-Cfw 4.30
PS3-Slim 120g CECH-2004b-Ofw 4.31
Still love www.softmodding.dk