Until you have issues that only your dealer can address. T4 units are not consumer fixable, especially the emissions components and the ECM.
You are correct in saying the available post 4 units are getting less and less on the market. Two reasons for that. One, people (like me) are keeping their pre 4 units and because they are hard to get, a used pre 4 units are commanding premium prices. I'll take my fuel efficient, mechanically injected stone simple diesel engines over a post 4 mess anyday.
been offered 30 for one of my pre 4 tractors that is 22 years old and that is about 10 less than I pad new for it. The way it's going, in another few years it will be worth what I paid for it NEW. That will never happen with a post 4 unit. They depreciate.
If Rudolph Diesel was alive today, he'd be pizzed at what tree huggers did to his engine.