I commented on pack weight in another thread a while back, but I'll say the same thing:
In my opinion (and I'm no expert, but I know what I like) the one single place I would not skimp is your pack. Without going all Gregory Baltoro on anyone, the options are excellent out there. I say anything under 3lbs (yes, I meant to type 'lbs' not 'oz' haha) is still plenty good for a thru-hike. I've hiked with heavier for shorter, but the one thing I don't want is hurting shoulders, trapezius, or other back issues. Just make sure the pack has the features you want (pockets, mesh holders, water storage/bladder hooks, whatever) and that it fits your gear, and that includes a bear canister.
People used to (errr...still do) hike the PCT, CDT, AT, etc., with all styles of packs, some newer and some older.
Find a pack that fits, find a pack that fits your style, find a pack that fits your gear, find a pack that fits your budget....THEN and only then start looking at ounces.
Again, my opinion, and one that may not be popular with some. Cut your tags off, cut your toothbrush, cut your hair, get a colonoscopy, but don't skimp on your pack for a 4 month hike. :)