The rule of thumb I have always heard was you go by tractor HP, not PTO. And then its 5 hp per foot. So a 4110 is 20 hp gear and 21 hydro. So either way it would be a 4 foot rotary cutter. I believe it would run a 5 foot too, but you would need to go slower in heavy stuff, or on a steep incline.
I run an LX4 on my 4010 with 2 less hp than a 4110. It'll power through stuff as tall as the tractor. The only bogdown I've experienced is if I drop it too low in really sandy areas and the blade starts digging in the sand. Then the PTO hydraulics slip it but don't bog the tractor.
My 4010 powers up my back hill with the brush hog going in grass; whereas, my neighbor says his 4310 strains to do the same thing. The 4010 is half the weight of the 4310 to power up the hill.
An acquaintance with a small 16 hp Kubota says he runs a 5' brush hog, but he's short of weight in the front without a FEL to offset it.
Do a search for "LX4 making fire breaks" or click on my alias and look for it at post history. Included is a picture of an LX4 in the woods hooked to a 4110. Wound up clearing a 40 foot or so swatch of tag alter/maple trees. A full four foot bite of thick stuff (over 8 foot tall, 3 inch dia) would slow it down considerably. Answer was to take less than a full bite. Would have been faster with more HP, but it got the job done. Good luck