I would try the 5' and see how it goes. IF you don't have to work it hard then the wider cut is a real convenience. This would mean setting it very high for the first cut of the season, minimal brush, minimal slopes to climb while loading it down cutting.
I bought the 4' rotary mower for my elderly 20 pto hp tractor. It is a good match except when mowing uphill on slopes, where I can't maintain 7th gear because the mower is sucking so much of the hp.
This year I mowed the entire 9 acre orchard with the 4' mower behind my smaller, 15 pto hp tractor. (also elderly, 25+ years old). It did fine except for even more sensitivity to uphill grades like in the photo. This was one of the few spots where I had to stop and shift to low range instead of rolling along smoothly shifting among the three speeds in high range. Also note the high cut at this spot, 4 to 6 inches. I had raised the mower slightly when I made the previous pass down that grade and got into that very dense patch of weeds.
If I were doing it again I might buy the 5' version and go slower. But my use is simply cutting weeds. For heavier cutting, I wouldn't exceed 4 ft.