I know the R7 is a stronger replacement for the R5. Also the R stands for 5mm crankpinand. In a N21-3T Basic engine there is a R2 mounted and could be replaced with a R7 so yes, I think the R4 has the same sizes.

I do believe the number indicates the strength because the R4 I have seen in older engines as I have seen one in a current offroad engine and the R2 was quickly broken when the N21-3T was tuned up to real performance.