Look at this link to jdparts. See is these parts could fit
See if the 4720 has a different turbo bracket. Both the 4720 and the 4120 have the same block and the same chassis, so if you want to buy a few parts, you can for sure make it happen.
It could easily be 2-3K for the parts though. What do you hope to accomplish? I would be surprised if you gained 5 hp.
As hot as the intake temps are I would have thought it would be more hp. Though I might add the debris getting stuck in between the cooler and the radiator might be a hassle. I just have to blow the radiator out now. The screen catches most of it. Not worth 2k. But I would like to think that the longevity would be better?
Have you checked the cost to trade tractors?