If you are "4 bagging it" one bag should be pretty much empty after cramming in all your regular gear. Around the world, winter touring or up and down the Americas may require partial filling of that extra bag. What is that extra space for? Water, multi day food stores or keeping the look of of rig clean by stuffing bulky hiking shoes on the inside instead dangling on top of the rear rack.
Best way to lighten your load? Drop down to two bags and a stuff sack. I try my darnedest to achieve this look while schlepping a two man tent. Cold/foul weather gear, lack of water hauling space often get in the way.
Yes, riding with a camelback -on your back- when you have four bags is a HUGE head scratcher.