you can
i just did....
first restore to the custom firmware but before you restore you have to PWNGE the Iphone by Redsnow, should be easy
then run GP and after that you will see the emergency screen after that you open redsn0w and click ur 4.2.1 firmware "orginial" then click to ONLY CYDIA and next .....
open ultrasn0w after.
then u are untethered and unlocked