Well, that's yet another spec.
My guess is that "mounted diameter" would be measured with no weight on the tires.
We are just guessing now, and nothing wrong with that... but my guess is that keeping the front to rear "mounted diameter" ratio the same for the new R4 as it was for the old R1s would be better than not paying attention to the ratios at all.
I assume you are going to do fronts and rears both... and that you already have figured whether the swap will require different wheel rims on both ends as well as tires. I've done a couple of swaps now.
Easiest thing would be to swap with another owner wanting to go the other way. I really like the tougher R4s. Also, R4s run higher pressures than R1s. So it is a stiffer sidewall running more pressure and with a larger wider footprint. More stable on hills, with noticibly less traction in snow and mud but more traction on hard pack dirt and in the rough.
rScotty