If its carbed(o-360) then how is the mixture set? is the E.G.T. guage accurate and in what cylinder?Has there been any mods to the cowl? The rear cylinders can run hotter.If the aircraft is a homebuilt like a lancair/glassair then over heating of the rear cylinders can be attributed to many things under the cowl.
not enough information about the plane and how the EGT is monitored. Long x country trips may need multiple adjustments to make up for air temp and angle of attack as fuel burns off. also humidity,of carb heat was left turned on,,,,,,,,