I have a Kioti CK20 with 22-hp and 15-hp at the PTO. The manual says max width for a rotary mower is 4' with manual transmission and 5' with a hydro. I asked the dealer about the difference and he said that with the hydro, even though it is less hp, if it starts to bog down you will let up on the pedal and slow down. Also you must mow in low range and it is not fast. So I bought the 5' mower and have never regretted it. Basically anything I can get my little brute to push over or ride over I mow over. Sometimes it sounds like a small war going on under the cutter, and I wouldn't want to be standing within 200', but I've only choked it down two or three times in the last five years. In a regular field it is pedal to the metal in low range ALL the time.
Now the CK20 is a slow little tractor with a two speed hydro. A tractor with more hp and weight might need the same size mower if it moved faster. But I am happy like mine is.
I would not waste my money on a 4' rotary mower. Not with your hp.
We use an old heavy duty Ford 6' behind a 52-hp Ford 4600. That tractor is twice as fast as my little one while mowing. I can cover a lot of ground in a hurry. But a smaller, lower hp, and much slower tractor would also pull the same mower. We have used a 10" behind the Ford but had to go very slow and it was not productive. Also hard on clutch engaging the mower.
I want to point out something else to somebody looking for a tractor to bushhog with. The larger the tire diameter the better off you are. The 4600's tires are shoulder high, the CK20's are waist high. The difference in ride across a rough field is astonishing. I can mow with the Ford comfortably at speeds that would be unbearable on the Kioti. I usually take the Kioti around the edges, pushing fallen limbs out of the way and tearing low hanging ones off with the loader. Have wife or mother (91-year old, can't keep her off the tractor) on the more comfortable Ford. Then I stay on the opposite side of the field from them because that Ford will throw stuff a longggggg way.
RSKY