I have a 23 kubota 3901. When I originally purchased the tractor the R4 where good for what I needed. Now we are going to put in a 4 acer garden for some ho.e use and retail.
I don't think the r4 will have traction pulling 2 bottom plow.
and don't forget R4's often have heavier sidewalls etc... aimed at loader / industrial type users.
R1's are targeted for ag. / higher traction applications and often have lighter sidewalls etc...
a swap to R1's won't change how much your loader will lift but tire failure can go up alot due to lighter construction as most R1's are not considered HD loader rated.
this may or may not be of concern if safe lift capacity is not the priority.