The Wheloon Prison wilderness area is the best source of backpacks for Tier 4 (heroic) Cultural collectables, and is a viable option for Tier 5 (epic) as well. Both runs are the same, with no fighting (invisi-run the whole way) but elementals might drop all the Tier 5s you need. Gargoyles no longer drop Tier 4 cultural, having been fixed at some point to drop Tier 2, so Wheloon is the clear winner for Tier 4 farming.
There are 20 backpacks total, but one is inaccessible under a pile of rocks. Of the 19 remaining, 17 lay on an efficient path. This guide lays out that path for the efficient 17 pulls in around 4:30. (That time can be improved.) There are also 3 cabinets for Arcane, but those just slow you down and Through a Mirror Darkly is a better Arcane farm anyway, so best to just stick to backpacks.
Here is a map of the path:
Here's video actually running it:
And finally, here's a printer-friendly version of the map, which is the same map but with inverted colors. Click to open, then right-click to save the image to your hard drive for printing with MS Paint.
EDIT: Some additional notes...
The two backpacks not in the mapped path are:
1) Just outside the Army of Shadow quest entrance along the northermost edge of the map. You have to open at least one, possibly two gates to get to it, both guarded by groups of mobs. So this one is definitely skippable.
2) Just a hair East of the center point of an imaginary line between "Q" and the shrine above it. This one should be reachable in a timely manner, but for the life of me I couldn't find a way to get there from "Q" so I just gave up on it. Looking again with fresh eyes, it's almost certainly inside the Oldstone Hall area, which is easily accessible if you head directly West from "N" using the short stone stairs to bypass the blockade. If so, grab it then head back to "N" and resume the path as drawn.
Looking at the video, it's 30 seconds between entering and exiting the bookstore ("J"), which holds one backpack. If you skip that one, the run in the video goes from 17 backpacks in 4:30 (16 seconds each) to 16 backpacks in 4:00 (15 seconds each), so it's probably better to just skip the bookstore entirely.