I suspect that for a V8 to operate somewhat smoothly on four cylinders, there is a very limited combination of four cylinders that you can use. Probably also the reason it goes to V8 mode below about 25 mph. Probably idle pretty rough on four but just my guess. I know before balance shafts, four bangers were pretty rough.

Just had another thought re: wear. Wouldn't the active four cylinders show more wear if they had some of this GM "piston slap" that seems to be talked about now?

 

Why does my 2009 Silverado stay in 4 cylinder Econ when it shouldn't? It doesn't happen all the time. I notice it after I travel 50 miles or more. Most of the time when I shut the motor off for a period of time I can correct the problem. My service garage thinks that I need a transmission control modular and or a solenoid replacement thats located in the transmission. The transmission specialist told me that this is not the problem.