I had a JD 3720 and now a JD 4520. Both had cabs and fairly loaded. The rear ends are completely different. The 4520 is a little longer and little taller at the hood.
The 4520 has significantly more clearance in the rear. The 3720 with R4s had about 8" clearance at the drawbar and the 4520 is closer to 15-16" with the R1 tires. However, the R4 tires are not that much shorter than the R1 tires on the 4x20 series. Bottom line, the 3720 with R4 tires, was not good in deeper mud. No traction and no clearance. This was probably the biggest reason I traded up.
The 4x20 has much greater stability on slopes. The 3x20 series is only 60" wide and seemed tippy. The has been noted my many other owners. The cab also accentuated the problem/sensation.
The 4x20 has more of an engine knock than the smoother 3x20. The noise of the Jd power tech engine is noted by many owners and is similar at idle and higher rpm. The 3x20 uses a quieter yanmar engine and noise is similar to kubota or NH/Case-IH. However, with cab, the inside noise is not significantly different between the JD series.
More power of the 4520 is more noticable when traveling in road gear. The 3720 would slow going up hills.
The 3 pt range of lift is better with the 4x20. This is in part due to longer lift arms on the 4x20. Easier to lift implement higher.
Both had the air seat but the 3720 rode harsher. Probably due to smaller tires, narrower width and harder sidewall of the R4 tires.
The 4x20 series has the optional self-leveling FEL which is not available with the 3x20 series. The FEL lift of the 4x20 is also significantly more.