well i am pretty much set on a triangulated four link, and that should be no different to setup than if the axle were below the frame. the bag position is not determined exactly and thats becuase of a few different things. what your talking about is a simple bag over axle setup which will give you the same travel from aired out to aired up as the bags usable stroke.

there are many different ways to mount the bag though, it can be behind and below the axle which still gives you the same travel as the bags usable stroke. you can mount it on the lower bars of your 4 link, which will give you a little better ride, and more travel due to the leverage between the bag and axle, but you will have to run more pressure to the bag. (i like this idea mainly because with the ideas i have for the bed, i can basically keep the bag under the bed and out of sight). those are the most two common probably, but there are so many different ways to mount it. i have considered a cantilever setup, , but i dont think i feel like the engineering.