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.
Avoid the 4425. They were made in Germany for the European market and a couple thousand were exported to North America during the 80's so Deere had a 4 row machine to sell. They are a decent combine. Problem is getting parts, very few in salvage yards and no aftermarket parts. Deere parts for them are VERY EXPENSIVE and more parts everyday have to be air freighted from Germany. A feeder house chain for one is a good bit higher than one for a 9500. With a little looking around a good 6620 or 7720 would make a nice combine for your acreage. Deere made many thousands of them and almost every wear part is available aftermarket keeping Deere's part prices down to stay competitive.