I believe the 6lug pattern is common enough on regular 'implement' wheels that you could just buy regular 15x10 or 15x12 6 lug implement wheels and use them. The 29x12.50x15 tires are readily available in turf/r3 or 'trencher' (look similar to r1 with shallower lugs) styles, but i have not noticed r4 in that size. I think r14 are available, though.
That's the tire size on rear of my b6100. Im considering going to 31x15.50x15, but undecided.