+1 Scree on the back support of the Porter 4400.
The disc between my L3-4 is actually ruptured to the posterior. To manage it I do daily exercises in which I arch backwards, forcing the back sides of the discs together, pushing the disc back into position to the forward.
This pack, and most well designed packs I expect, loads the spine below the lumbar section pushing it forward, while loading the shoulders so that they are pulled backward, creating a healthy (at least for folks like me) backward arch of the spine.
I am dialing in the pack for a 18 day off trail expedition trip in September, with an approximate weight of 45lbs. Yes, backpacking light can get heavy when the trips get long with no re-supply, and a shotgun, and other things, are necessary for safety.
It is carrying the load better than I could ever have dreamed.