I believe a 4425 was made in europe instead of North America, and there weren't as many of them, so used parts aren't as available as the 4400-4420 which used a lot of the same parts over a long model run.
I have heard horror stories of long delays getting something for one.
Main thing is buy something in good shape or be prepared to work on it. A larger machine might be cheaper or just as cheap and more choices out there.
Do a search here for 4425 and you will find a lot of discussion about them as well as a serial number by year listing for various JD combines which might be helpful.