Should I trust the tire pressure sensor in my truck or a hand held gauge?

My tires looked a little low. The truck gauge read clockwise from driver front: 33, 34, 33, 32 psi. I checked it with a hand held tire gauge and got 29, 30, 29, 29. I borrowed my neighbors and got the same readings: 29, 30, 29, 29. So I filled all the tires up to 35 psi according to the hand held gauge. The truck automatic gauge read 39, 39, 39 38. So it appears that it is 4 psi different from two separate hand held gauges (one a stick, one a dial).
Which should I believe? The one built into the truck or my old gauge that I have used for years?
Any thoughts?