I picked up a used #4 last winter. The critical section where the shoulder straps attach to the bag looked too stressed for my comfort. After breaking 4 upholstery needles and nearly spearing myself through i gave up.
One possible solution would be to ship the pack out to someone like   (You need to Login or Register  but with a minimum 3 week wait and shipping costs it's not a good option right now.
I swung by the local shoe repair guy; he got that thing under the sewing machine and went back and forth a number of times. It's a rock solid repair for a whopping $6.00 (cash only).

Another option for your heavy sewing needs is the local auto upholstery shop. Like the shoe repairman, they have equipment that is stout enough to drive heavy thread through multiple layes of thick webbing and/or material.

Pat

I'll add one more option if you're in horse country.  Try your local tack shop.  See ya'll in the Q next month!

Portage Dog