I used to have a 6' rear finish mower on a tractor about the same size and slightly more powerful than your 4200 (mine had 23 PTO HP, yours has 20 or 21) and I used it to mow my lawn several properties ago. There was a big sticker on the top of the deck that said in large, capitalized, bold letters "LAWN GRASS ONLY!" This is absolutely the truth, you only want to use a rear finish mower for mowing a lawn. You will destroy it mowing either material or terrain that you would not use a zero turn or any other sort of regular lawnmower to mow. Generally "uneven and rough terrain" does not equal "lawn" so I would be very careful in getting any sort of a finish mower for "uneven and rough terrain." If your terrain isn't really "lawn," then get a good bush hog and you will be happy. I now live elsewhere and have a much larger tractor running a Deere MX6 and it gives a surprisingly good cut quality along my gravel driveway and ditch where I would ding up too many finish mower blades to want to use my zero turn to cut. The 23 PTO HP tractor I used to have would run a 5' bush hog just fine so your 4200 should be able to run one such as an MX5 well.
If it's really not that rough or uneven, then a rear finish mower would be fine as long as you have wide open areas (several acres) with few or no obstacles. A rear finish mower makes a tractor pretty long and unwieldy to do any sort of mowing around obstacles. I also noted that only 23 PTO HP was not enough to run the mower well, I had to mow at a maximum of 2 1/2 MPH or I would bog down the tractor with 4-7 days of lawn grass growth. You have even less power than I did, so I would absolutely say avoid the 6' model and use a 5'er. A rear finish mower isn't hard to connect as it is simply a PTO-driven 3 point implement and hooks up the same way as any other. The one caveat to a rear finish mower is that it rides on its own four wheels so height adjustment requires lifting the mower and adjusting the gauge wheel bushings, which is kind of a pain in the butt if you ever change cut height. Some mid mount mowers are like that too, particularly on larger tractors. A mid mount mower makes the tractor a lot easier to maneuver around obstacles than a rear finish mower but they can be a pain in the butt to attach and remove, and makes your tractor difficult to do anything else with when it's installed. Mid mount mowers are also proprietary and notably more expensive than a rear mounted mower, you can buy 2-3 rear finish mowers or one decent enough zero turn for what one mid-mount mower costs.
I will close by saying that I generally don't recommend a tractor larger than a subcompact (which your 4200 is not) to cut lawn grass with. A tractor is hard on a lawn in causing ruts compared to an actual lawn mower like a zero turn due to its much higher weight. You also will tear up a lawn even worse if that tractor has anything other than turf tires. However, turf tires are notoriously bad for anything and everything else you'd use a tractor for. I recommend a tractor with fluid-filled ag tires for "tractor" tasks and a zero turn for mowing.
