used pwnage to jailbreak my 3gs (old bootrom) successfully to OS4. but cydia crashes each times its launched. now im stuck, cant run ultrasn0w to unlock the phone.
sound strange, I just upgraded and I am in the same situation as you, 3gs old br, from 3.1.2 to 4
I would start again, downgrading to 3.1.2 w/o jb and then pwnagetool to go to 4