I had an overheating problem with my '86 300E a year or so ago and it turned out to be an aux. cooling fan that was wearing out. Here's a quick test you might try. Get the engine temp to the point where the aux. fan comes on. CAREFULLY press in on the center of the fan hub with a piece of wood dowel or the like. If the fan speeds up noticeably it needs to be replaced. I'm not sure if you're relieving friction from a worn thrust bearing or making better contact with the commutator brushes by doing this, but either way it indicates an internal failure that can't be serviced. I replaced my fan and the engine ran much cooler, especially with the A/C on. My fan had about 95,000 miles on it at the time I replaced it.
I'd be surprised if your radiator needs replacing after only 4 years. Do you use distilled water to mix your antifreeze?
I also switched to Evans NPG+. Great stuff! Nonaqueous propylene glycol, non-toxic, 375 deg F boiling point, system runs at less than 2 psi pressure and they claim it to be good for 300,000+ miles between changes. Because there's no water in it, there's no corrosion. I've had it in for about 35,000 mi. so far with no problems, and I've put it through some very hard use.
Hope this helps,