Okay,
There is a thing called lead and lag. What these terms refer to, is the front tire's relation to the rear. If the fronts cover more ground to one revolution of the back tires, then you have a lead. If the opposite is true, you have a lag. On a MFWD type tractor, the prefered is to have a lead of about 0 to 2%. The main reason is that you have better performance if the fronts are pulling ahead all the time. If you have a lag, you are pushing the front tires with the rear, and that sort of kills the benifit of having 4wd.
There is absolutely no reason you can't run a turf tire on the front and an R-4 on the back. The key is staying in that 0 to 2.5% lead. The industry accepts a 2.5% lag as well, but I believe the optimum to be in the lead side of zero.
One key piece of info is knowing what the ratio is from the front to back. That will tell you what you need to know. I have a 3520 and I know that the ratio between the rear and the front is 1.623. So that means that for every one rev of the rear axle, the front turns 1.623 times. I got this info from JD's tech people online. Took a week for them to get back to me, but they did.
The other thing you need to do is know the Overall Diameter of the R-4 tire you are taking off, and compare it to the turf tire that you are wanting. If you can find two the same, then you in business. If you can't, then that is where the fun begins. Trying to find some rear tires that will match up to your fronts.
I will use my 3520 as an example: The front tire is a 27X8.50X15. So, it is 26.8 inches in OD. If I take that X 1.623, I get 43.49
That tells me that I need a rear tire at or a little smaller than that figure. The rear tire is a 43X16X20 and it is actually 42.6 in OD, so I have a lead, of2.1 %, meaning the front tires are traveling 2.1% further than the rear tires are in one revolution of the rear tires.
To answer a point brought up earlier. The reason you have a 5% (+or - 2.5%) window is to take into account tire wear. The fronts will wear about two to one over the rears depending on how much you run the it in 4wd. So as the tire wears down, you give up a little lead.
So it is doable, but the issue is finding a tire that will give you the OD you are looking for to keep your lead that you need.
I hope that made sense. It is late and I am tired.
Good Luck.