Roy, just curious why did you sell the tractor?
Couple reasons:
Curtis Cabs are OK...better then nothing and factory cabs weren't too common when the 4520 was sold in 2005
However, if you have to do work behind the instrument panel, the cab has to be removed or, at least, partially disassembled. For example, I needed to replace the fuel tank sender and the cab would require removal (per the dealer. My "fix" was just keeping the tank full
The Curtis cab had a lot of openings to the outside. In defense of Curtis cab, there may have been pieces (gaskets, corner pieces and such) installed initially (in 2005). Even with a heater, it was pretty cold and drafty.
So, a better cab was one of the reasons
No AC available (at time of installation), but the doors could come off and both the front and rear windows could be opened...
So, lack of AC was one of the reasons
Mechanically, the 4520 was great. The Deere Powertech engines are cold natured, but it did have a block heater which was used anytime the temperature was less then 30-35 degrees. So much easier on the engine. Deere uses a Yanmar engine in the newer 4xxx series tractors. I've had three Yanmar powered Deeres over the years...all started easy, even below 30 degrees...but those were all garage kept too. The 4520 (and soon to buy 4052) are too big for may garage
The 4520 was showing it's age, cosmetically...which didn't bother me. Problem was, I'm showing my age (almost 71) and comfort is a bit more important now.
If I could have kept the 4520, and bought a newer cab tractor, I would have had the Curtis Cab removed