R4 is just the commonly used model name of the compressor. There's an R6 that was popular for some of the other US cars, for example. It's a little misfortunate that it appears similar to refrigerant type numbers.

For my part I like freon detectors instead of dye. It works well, is quick and easy to do yourself. Check EBay for freon detectors and you should find them under $100 and you'll have a tool you can use a long time on many cars.

If your compressor has locked up, you are looking at a much more extensive repair process than just checking for a leak. You would want to do a compressor, receiver/dryer and probably expansion valve replacement. And a flush. And a vacuum pull. Then refill with freon. Some people suggest replacing the rubber pressure lines too on the theory that hot metal particles from the failing compressor may be embedded inside the rubber tube.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles