There is a few things to know about them, might have been mentioned below but if not..
Keep the oil clean in these, good idea to run the carb out of gas when done with it, and the diesel is spinning. They like good spark and fuel, have to keep that magneto firing a nice spark. I am not sure if the carb on this horizontally opposed engine is prone to leak down fuel into the crankcase, I have seen it on my D7, overflowed the crankcase, run it like that and you already know I am sure LOL.
Some of these need to have the diesel engaged to circulate water, my D7 does, not sure about the D4, so if working on just the starting engine, you can't run them long, again, I am comparing apples and oranges, but... good to know.
These are great in cold weather, if you can get that starting engine running, then the diesel is likely to fire, might take some spin time prior too, but no batteries and electric starter in my opinion, on one of these will compare, especially with an engine that has some wear, less compression, etc, sawmill near me has a D13000 (D8 motor- 40's era, no turbo) and without that starting engine, cause its worn, be a real problem for him to get it running, its great, you get oil and coolant circulating, warming, no compression, half compression, and the motor will be well lubed and warming up when you turn on the fuel, unlike a cold start up, you don't get that, this I am sure is just redundant to mention, but I am a fan of these for this alone on these older ones.
One of the reasons people say they are a pain is because no one really takes care of them, I can say, that the one D7, I could hand crank the starting engine and get the diesel to fire, with little trouble. They are a seemingly old tech, high rpm engine, but with some care, they can run pretty good, the one on my D7 will really lug down in the cold, takes awhile before you can put compression to half or full, hard to believe it was also designed for the D13000, (D8 motor, 6cyl.) given how it just seems to be enough for the 4 cylinder D8800. Often times the old gas tanks are rusted and have coffee grind size rust particles plugging things up, like the sediment bowl, that can make for a poorly running engine.
I do believe I have one book in my military tech manual collection which is for a 40's era D4 tractor, I would highly recommend your friend sourcing the Serviceman's Reference Book, Operating Instructions, and the Parts Catalog, my tech manual has all of those bound in. I kinda collect these just to have and for reference and helping others, it has all those books bound together, if in fact it is of a WWII era tractor, these are nice books to find for those tractors, they do appear on ebay, but I have been outbid by people who use them for reference just for making models, they used to be inexpensive, but are really nice finds, just not so inexpensive any more, they are a pain to search for because not everyone lists them with the same title etc. Over the years I have been able find various ones for that era of Caterpillar tractors There was a person who was on another forum/board and he listed one, first one of these I ever found for my D7 and he was real greedy about it, so over the years I grabbed as many of these as I could find, for the purpose of eventually, not as of yet though, selling the extras off and making my money back, I do not like people who know you need something, its a one of or hard to find and they want too much money, knowing you'll go for it, because where or when you will find another etc.
Hopefully some help, I think often times since some of these tractors are so old, not everyone knows about some of the important things when using the starting engines, how to take care of them etc, probably straight up for you with some helpful tips.
One thing about that tractor, if it has a 4 bottom roller track frame, they can nosedive a lot when cutting, the 5 roller frame is better, your friend may or may not benefit from learning on one of these, can be difficult at first to take an even cut, loose material and pushing that off is easier to learn on as you probably already know.
Serial number will be on tags in 2 places and did match from the factory, one will be on the back, left side of the tractor, the other on the engine block, if no tags are present, it will be stamped under where they were, if different numbers, means the motor has been changed out. For example;7U is one series of these and is a wide gauge, 6U is a narrow gauge. There were 7J, 2T and 5T series of these as well, might turn out to be one of those if that old.
