This may be oversimplifying, but what does the Plus Four weigh (as driven) in pounds, divide that by the tyre pressure and that is the footprint that the tyres need to have in contact with the road to make the equation balance. Reduce the pressure and the footprint has to increase to keep the equation balanced. Increase the footprint beyond what the designers (both car and tyre) intended and the performance of both will be compromised.
Stick to what MMC recommend IMHO.
