I think you might be misunderstanding what they are telling you. Your fifth conductor is assumed to be a bonding conductor between the panels. If the panels are connected with the proper metallic nipple, locknuts, bushings, etc., this becomes the fifth (bonding) conductor. The other four become A,B,C and N.
The sub panel's neutral bar IS NOT bonded to the can.
Now, if you are using a non-metallic connection between the panels, then you'll need a fifh conductor for bonding.
The only exceptions to this are a few rare ones when dealing with detatched buildings.
