The 4 acres per hour was doubtful to me so I did a little test today. I bush hogged .9254 acres in 45 minutes. Measured the area with a gps. It was open pasture with a few trees to mow around. I was running at 1900 rpms in low 3rd gear. I was running around 4.5 mph. Fairly short grass that was mowed late last summer.
Never had to touch the throttle, never backed up and mower was always in the grass until last few passes. Conditions rarely get any better than this and don't really like to go any faster than that. So under excellent conditions my best would be about 1 1/4 acres per hour. 49 hp tractor with 6' mower on wide open pasture.
4 acres per hour with a 6' mower would win the gold at the tractor Olympics and could compete with a 15'batwing