I use SD! and have been very pleased. You may be able to partition the external into 4 partitions of equal size for the 4 computers, but they would have to be large enough for the biggest HD you want to back up. SD wants to see a FW400 connection. Looking at your signature, I don't think you'll be able to do that, though. You wouldn't want all of your backups on the same external anyway, would you?
I suggest you go to the SuperDuper! site at http://www.shirt-pocket.com/forums/forumdisplay.php?f=6
and ask the question on the forum. They are friendly and very patient people.
The bootable drive feature is excellent. You should be sure that each of your computers' HD names are different from each other, and then name the backup partitions to match the names. To start from a bootable external drive, just hold down the OPTION key when starting and all available bootable drives will show up on the main computer. Then, of course, you have to pick the right one. To backup though, just run SuperDuper! and select the original and the partition for the backup and off you go.