Hi phil, while I was running 7.50x16 I simply ran them at the max pressure indicated on the tyre. Less rolling resistance but not very comfortable. I think you lose grip when over pressure but my Daily has never gone fast enough to find out
I don't know how far back the HUBB archives go but I think a pressure/load/conditions data sheet once got put up here a few years ago.
I'll try and remember to dig up the version I downloaded.
My rear twins are not a good example, but on the front, axle weight of 2 tonnes, I run 235/85/16 tubeless bridgestone M/Ts at 4.5 bars on the road, 3 on stony pistes and right down to 1.5 in sand (but not for too long)
You should be able to run your van (4 tonnes) at 3 - 3.5 bars without the tryes looking too squashed.
I don't know whether retreads put up with low pressures as well though, so keep it up (so to speak)