I have used remastersys on BT4 Final twice and both times it resulted in a backup ISO that was not valid.
And, after each use of remastersys, I found that the remastersys program was no longer installed in BT4 Final.
Had to go into synaptic package manager and re-install remastersys 2.0.12-1 each time.
In my testing, I am using a full BT4 Final o/s on an 8GB usb with the remastersys config set for the ISO to be placed on the hdd.
EDIT: An exact duplicate of the entire process above was done with Ubuntu 9.10 with no problems. The remastersys forum believes there might be a BT4 issue involved. With BT4's use of Ubuntu 8.10, the correct version of remastersys (2.0.12-1) is being used in BT4 (confirmed by the remastersys forum moderator). I hope someone with a hard drive install can test the remastersys process and provide their results.