It depends so much on tyre type and size, and what vehicle you have that it is impossible to make any particular rule as to what pressures you should run. I read the 4PSI rule, and I agree it is crap as well.
With my D40 Navara with 31" MT ATZ's I ran 40,42,40 (front, rear, trailer) on the blacktop and when I hit sand I would drop them to 30, 32, 30 and go anywhere. Only once I had to drop them lower and that was only to about 24 PSI.
Now that I have my 79 which is a heavy bugger, with 33" MT ATZ's I run 46, 48, 40 on the blacktop and on sand I have to go down to 16, 18, 20 or I'll get bogged in the first 20m.