When I ordered my TC 30 I inquired about turf, r4 and ag. For each tire type you need to change rims. It gets pretty pricey. That's why I sold my other tractor. For what I sold the tractor for and the $2200 I would have spent to change my ags to r4's at the time, I bought a new tractor.
Tire sizes are:
Ag. Tires 4x4
Front 7-14, 4pr r1
Rear 11.2-24, 4pr, r1
Turf 4x4
Front 25x8.5-14, 4pr, r3
Rear 13.6-16, 4pr, r3
R-4 4x4
Front 25x8.5-14, 6pr, r4
Rear 15-19.5, 6pr, r4