Following from Caravaning 2 axle / 4 wheels

Nope, never heard of it!

Can't think of any good reason for it, but he may have a good reason, ask him!

snailbait
Sounds to me like he might be a bit concerned about your ball weight. By raising the rear slightly he is putting more weight forward. I can't see that 4psi would make a huge difference but it brought to mind the rorts that some of the grain truck drivers used to pull with airbags under their loads and pumping them up/letting them down as they went over the scales.

teege

You have not told us much. What is the van?

Just a wild guess. I notice you are towing with a Prado. What type of WDH are you using? If you are not using the Hayman Reese system but using the goose neck that came with your Prado, your ball may be too high for your vans chassis to be parallel with the van. If your van does not have weight sharing suspension then the rear wheels will be carrying significantly more weight than the front - hence the recommendation for extra air in the rear tyres.

I have noticed quite a few vans travelling along the road that are nose high. In nearly every case they are towed by a 4WD without WDH or the WDH is only the shepherds crook type (and are far too light for the van being towed.) A considerable portion of these vans have independent suspension that is not load sharing (ie they do not have Simplicity suspension.) Those vans will have a lot much more weight on their rear suspension and I wonder if some of them have overloaded rear tires.

PeterD