When I got my JD 3720 cab, the ride did seem rougher than my prior Kubota 3710 and I have the air ride seat. The tires are smaller in diameter and the stance is narrower which account for some of the difference. I believe the cab also gives the perception of a rougher ride because is moves/sways as you drive and the visual perception may also make it seem rougher. I have become accustomed to the ride and now it is not as noticeable.
Fluid filled R4's will ride slightly more harshly than air filled but the R4 is still the give a rougher ride compared to other types of tires. Wheel spacers probably would have little benefit for the cost of the spacer unless the spacer was 6" or so on each side. Are your tires turned out to the widest width?
Fluid filled tires with lower air pressure may have a marginal benefit but wouldn't hurt to try but don't get pressure too low.