Tim:
I have a 2001 4200 with about 675 hours, and it's been a good rig for me.
DFKRUG is the absolute expert in my opinion on the 4200 - 4400 series tractors.
I learned from Dave that the rear axle housings are cast aluminum, so take a look at the 3PH mounting ears.
I had the battery issue with acid corroding the hydraulic system radiator to the point of failure, which was fixed under warranty on mine. The biggest problem was the Deere batteries of the time leaking acid. Newer Deere batteries are sealed units...finally. The battery is an odd size which means you go to Deere for a battery, or you fabricate new mounting brackets for a common size unit.
Check out the linkages for smooth operation. If this is a HST unit, finding neutral can be tough, and is the nature of the beast.
I like the plastic hood and fenders because they won't rust.
The rockshaft filter mentioned earlier is under the seat on early models and above the PTO guard on later models. It's a sintered bronze unit in a black metal housing about 3" - 4" long by about 1.5" in diameter. If you buy the rig, take it apart, clean it, and assemble it back in the right direction per the manual.
As others have said, the loader masts can get wobbly with age at the mounting tube attached to the frame. DFKRUG shows how to remedy that in his 4300 rebuild. Do a search, and you'll learn more about this series of tractors than you imagine.
If you buy it, buy the shop manual on CD from JD online or via your dealer. It's half the cost of the paper version and you can print out what you need.