I have had a 4010, 4020, and 4230....newer model 4020. Two were diesels and one LP.
I currently have a 4 year old tractor I bought new with a Cummins 3 liter naturally aspired diesel. HP is 65 engine, 57 PTO. Fuel efficiency on the new tractor is much improved from the old ones. Reasons could be technology, or the fact that the new one is just below the 10, or because it's 4wd. Don't know.
I also currently have a couple of '60's Ford diesel utility tractors in the 30-40 hp range and they get exceptional economy.
So I have the best of both I guess.
On the new one, the rearfenders are fiberglass, as is the top of the cab (my deere was plastic too) of the quality of a fine boat but the rest of the exterior is steel. The cab interior is part metal and part plastic. The plastic is non functional facade.
I like both.