downgrading/restore question

i have two iphone 3gs
first one is bought in this summer. running 4.0, i saved SHSH in cydia and i also learned how to restore the phone without getting upgrade by itune

the second one i just bought from ebay is iphone 3gs "4.1", which is not yet jailbreakable

Im wondering if i can use the method above(restoring the phone by using 4.0 firmware from modmyi) to downgrade my new iphone 3gs 4.1 ios.