I wanted a 5ft rotary mower but was talked into buying a 4 ft by my neighbor who has farmed here for two generations. The mower dealer, a VN import house, also told me 4 ft is what they specify for a YM240/YM2000.
My actual experience is that 4 ft is all my YM240 wants in tall wet dense grass and going uphill. I have to run in H2 going up the slope but can manage H3 level or downhill. That's in grass 2~3 ft tall like in the picture.
If you haven't let the grass get that tall or go slow for the first pass of the year, I think it would strain a little with a 5ft mower but of course you would accomplish more per pass.
If you can get an excellent price on the 5 ft mower, and don't have much slope, I would do it.
This mowing in wet grass is about the heaviest load I've worked the tractor. It's at least as much load as discing. I've never overheated it but this makes it plenty warm. That's the 4 ft mower.
Don't worry about lifting it, no problem. (aside from you will need front ballast).
Horsepower: YM240 means 24hp American measure, which is the naked engine on a test stand. The identical engine is rated 20 hp out the PTO, Japanese measure, so over there they call the same tractor YM2000. All(?) Yanmars have the hp first in the model name.