Securely wipe all user data from iBook G4 / OSX 10.4?
You can take your 10.6 knowledge and apply it down all the way to 10 as the big changes in user level data and file system changes arrived at 10.7 and later. 

The easy way out, is boot the Mac in target disk mode and connect it via FireWire to any Mac and zero the drive, secure erase, etc... The recipient will likely be able to order a 10.4 replacement disk for less than $25 online and set up the OS as desired. 

Also your plan to just make a shell user account and delete the old user accounts would work well for light protection. Better to copy movie files or other data over all the empty space then delete them to prevent a simple undelete from recovering files only deleted by the OS. 

Maybe examine the system keychain for any wireless passwords you don't want escaping, but most things of a sensitive nature are in the user home folder.