I rented a 4310, and it is a nice piece. A little underpowered, but very nice. The key advantages I see with the 20's is:
Larger fuel tank (13.5 gallons) This is nice considering what a PITA it is to "can" fill a center mounted tank.
More power (44HP!) from turbo (3520 and 3720) and intercooler (3720
Better visibility from curved hood
As for tire size, that is same, BUT there are 2 sizes of R4's available. I went with the larger ones which are about 10% larger at 43x16-20, and 27x8.5-15 because I wanted something in between the 4000-20 and 3000-20 and this 10% bump in tire size got me there.
As for cabs, the Jodale Perry
www.jodaleperry.com looks like a VERY nice unit. The 4000-20 unit is about ready, and uses Power Beyond to run the AC via a hydraulic motor.
I looked at the 4000-20's, and aside from being too big, the departure from Yanmar, and utilization of Stanadyne for the fuel system turned me off. Not only do they smoke hard at initial start up (like most all compact diesels do) the Deere/Stanadyne systems rev up really hard instantly but briefly...
Any info on Laurin's Eclipse would be appreciated.