shrinking a partition on a thumb drive

[Lenny]

Hi,
I am running Ubuntu 10.04, and I have it installed on an 8GB thumb drive as 
well.
I boot from the CD, and unmount the thumb drive.
I open gParted, and select /dev/sdb1 which is correct.
Then I go to move/resize and I change the unallocated size to 4096 MB.
then when I apply the action, it fails every time.
I tried this in Parted using sudo as well, and it said that the action is 
not supported at this time.
My goal is to make a 4GB partition on the SD card.
Thanks for any help.
Glenn