I have a MountainSmith Specter which was the ultralight backpack of choice in 2001 by BackPacker's magazine, it weighs about 4 lbs and has the weight and capacity features you need. Only been used twice -- no more than 2 weeks's amount of use. In super, superb shape. Will sell for $70 plus buyer pays shipping. (New was $270).
PM me if interested.
Mountainsmith Specter (5600ci/91.8l, 4lbs 5ozs) - For winter camping and mountaineering, this is my new Yeti pack. Born of the weight-friendly older Mountainlight series, the Specter and its smaller siblings are about the lightest, strongest packs you'll find if you require a full suspension system for heavy loads. If you have faith in Backpacker Magazine, their 2001 Editors' Choice awards were given across the entire Mountainlight-derived series.
Mountainsmith Mountainlight Packs
These are the most comfortable lightweight load-carriers we've tested.
A nice forum post on this pack:
"J R ( RavenUL ) not the lightest... but my fav! on 04/01/2005 12:13:13 MST "
"I'm a big fan of MountainSmith. Their MountainLight line isnt the lightest out there, but they have a VERY functional waistbelt, good frames, and still have a good weight to CI ratio, and a good durability to cost ratio.
When carrying anything more than a base weight of about 10lbs (or longer than 3 days), I go with my MS Ghost. Its an excellent pack, but maxes its comfortable weight at about 35lbs, however for heavier loads their are a few beefier options - I like the Auspex, and it sounds like it would fit your needs well (light base load, but need to haul alot of water). If you foresee a need for an even beefier pack, the Specter is a very good pack as well - with a quick detach day pack for side trips if you so wish... both of these packs will ride well with 45+ lbs loads, but weigh a good 2 - 4 lbs less than other framed packs of similar dimentions and quality."