Ever since getting the PMDG 747v3, I have noticed this.  It's not specific to P3Dv4, as it happened in v3 as well.  Anyways, after turning on the packs and establishing AC, I set the Pass Temp. Controller to set a Master temp. of approximately 71-72F and it works beautifully on the ground as soon as turning on the packs after APU start.  After a few hours in cruise, I will pull up the ECS Page, and notice that the master temp. command and also the ambient temp. is now 75F, without having touched the controller.

 

 Now, this only happens with the passenger cabins and controller.  I can "reset" it by turning the Pass. Temp. Controller by 3-4 notches and then returning it back to the original position I had it in and the temps. will go back down to 71F.  However, after a few more hours enroute, they'll be back to 75F somehow.  The Flight Deck stays at 71F as it should.  After watching this a little more closely, it seems as if during cruise the temps. will creep up in 1 degree intervals.  It's really only noticeable on long hauls, as it happens very slowly.  Anyone else have or notice this issue?  I always fly N127UA(B747-400 w/ Pratt) if it matters somehow.

The temperature rise from set point after an elapsed time is a feature and is explained in the FCOM.