Talking with a fellow hobbyist, whom is a real life pilot, and also has flown in full scale aerobatics, and now is into IMAC, he and i have made some possible reason as to why the rear cylinders are cooler than the front two.
The Shroud from Piorun is made so that in the front it is bigger/wider, than the rear aspect.
as a result of this, the space between cylinders and the shroud is way bigger in the front and much tighter in the back 2.
this has an effect of the air, in the front being in a higher pressure, and thus, less flow. towards the rear, the air pressure is lower, due to tighter area, and from the effect of the higher pressure in the front, this increases air flow to the back.
he tells me, that in real air cooled pistons airplanes, there is a baffling system or as he describes. plenum, in which the distance from baffles to cylinders is same in the front as the back. this creates an equal air pressure throughput, and thus, and equal air-flow. Cooling, in theory, should be the same. My question is, how many 4 cylinder quads out there, in model airplanes, have all 4 cylinders with the same temp???
i am guessing, probably none. My statement being, fly the airplane and listen to the sound of the motor. if it is running well, leave it alone!!