We had both a 4830 & a 4038, which was really a confusing JD nomenclature deal. 

The 4038 is a great sprayer. No problems. Not sure what the topography is like where you are, but the engine/hydraulic system was just fine in our area. Not very often that I even used full throttle if the load didn't demand it. Ours was a 2014 model, bought new.

Boom rust occurs. I don't know if it's from spraying UAN or AMS, or both. But ours had some corrosion. However, it doesn't seem to bother anything. I believe some owners have disassembled the boom and had them repainted. Unless you're a real young guy, I'm guessing it will outlast your lifetime anyway. I did not see any structural weakening on our boom from the corrosion.

Now I understand that if they had a dry spreader box mounted at one time, that's another concern. Look for deeper corrosion back in the frame. We never had a spreader, but had heard that there could be more problems than just the boom. I assume this applies to any mfg, not just JD.