I have been a user of garritan products since GOS was first released. I have attempted to upgrade to GPO4 but none of my serial numbers are accepted. I must not be entering the right number and I'm obviously doing something wrong. I have entered the Serial numbers in the case of GPO, JABB, The Strad and Gofriller Cello and all of them were turned down. I also tried all of the numbers that I wrote down as registration information at other times. Nothing seems to work.