Mine won't stay in 4cyl mode unless going down hill. Even on a flat going 60mph it will only do 4cyl mode for a few seconds at a time. My buddy's truck can do it for miles and his is stock.
Can you get the ECU flashed or hyper tech chip or something to hold it in 4cyl mode more? I can't step on the gas more than about 1/8" and it kicks into V8 mode. I've love it if the 1st 1/2" (up to 1/4 or 1/3rd throttle) of pedal movement.
Mine is generally the same way! I just think the wind helped me that day or something... But my tune is performance not economy...never tried that to see if it would help any