using D4 4g parts on D4 5t cat...

Welcome to ACMOC, congratulations on scoring a old D4, from my limited experience of owning a 4G D4 and a 2T D4 there are a lot of parts that will swap straight in including the main motor, pilot motor, transmission and most undercarriage parts like rollers and track chains, there were subtle internal and external changes in those older D4's between 1936 to 1945 and I assume the 4G is a narrow gauge tractor but I think you are onto a winner with your game plan. We look forward to seeing some photo's.
Good luck with the restoration, those old D4's really have a nice bark when they are working hard.
Regards
Mike

I wouldn't say there was that much interchangeability. Many changes during the 4G model run and although assemblies will interchange with the 5T a lot of individual parts will not.

If it is simply a diesel engine swap then the various inlet / exhaust and water manifolds on both the main and pilot will switch between the 4G and 5T engines won't they Pete, there is obviously some difference with the firewall and throttle linkages.

To many differences to list. Also depend on s/n's. Manifolds are different, governors are different, injectors change from clamp down to threaded, track trucks, carrier rollers are different, some s/n sprocket part numbers change etc.
Internals are fairly consistent. Study up on the parts books and see for yourself and actually compare s/n's of what you have.

I agree Pete that a lot of the later 5T parts are different to the 4G but am I wrong in assuming both models at their core share the D4400 diesel engine and that the bolt on parts such as the head, water pump, fuel tower and governor, water, inlet and exhaust manifolds will transfer from a 5T back on to a 4G main motor because that was the question I thought was being asked, if the 5T engine was possibly ruined from water damage could the tractor be saved by using a local free 4G D4 short motor as a donor, or if not being a purist, could you simply bolt a complete RD4 engine with radiator onto a 5T transmission, is that possible?

As I stated, complete assemblies would interchange but individual parts particularly on the engine may not. Yes a complete engine would interchange. Parts books and s/n's would tell the story.

Complete units can be swapped round but the track frames don't swap because the final drive outer bearing hubs are different.