I like McHale and though pricey, suggest you give Dan a call. If you are in the Seattle area you can pay him a visit for fitting and getting a "test" pack. If not you can do it by mail.
As part of his process you'll drop a deposit (all or maybe only part of it non refundable if you don't buy) and he'll send you a "test" pack to try out so you can see if what he has to offer will meet your needs.
Me, I have 2 of Dan's packs. Both packs have his "bayonet" system which adds 6-8" of frame above the shoulders to steady/handle big loads.
A 20 year old Inex, weighs 8 pounds and holds enough gear for just about anything. I used it as a family sherpa, carrying gear for me, wife, and 2 kids on weekend long hikes (used just about all its volume), 2 week solo hikes (used about 1/2 its volume) and myriad hikes with the Boy Scouts where I was the responsible trek leader (used about 2/3 its volume). The heaviest load I carried was 80lbs. The load was heavy but the pack carried it well. No chafing, no sore hips or shoulders, no pain at all. Just a grunt.
And a 5 year old Sarc-chasm. Gotta love the name. This pack is maybe 50% of the volume of the Inex. I've never used the bayonets on this pack. Weighs 3lbs no bayonets/lid, maybe 4 with. Heaviest load carried on a 10 day Grand Canyon trek is 55lbs. The pack easily handled the load.
The strength of a McHale pack is three-fold. First is the frame. Dan uses a real frame that is strong enough to carry big loads, and is stiff enough to balance the load between the hips and shoulders. Second is the hipbelt. Dan's hipbelts are about 5" tall and use 2 buckles. The hipbelt masterfully distributes the pack weight without pinching, chafing. Very comfortable. Third, is the custom fit. There are a few other custom pack builders who I am sure offer the great fit, but don't have the great the frame and hipbelt.
If you want UL, Dan will craft a pack bag of cuben, but I think for 4 pounds you can get a great pack, well suited to your needs. When you go to the mchalepacks website, look for the Sarc packs as I think these as intended will meet the needs you outline.