I was the one who posted the mention of a Sanden compressor. I have two Euro 240D's, one a parts car with these compressors on aftermarket acir conditioners in the same location as your R4, low on the drivers side. The R4 is a terrible compressor.
As Steve said, when you replace the R4 use a NEW, repeat NEW, R4 compressor, not a rebuilt. If you do that, there is probably not a need to convert to a sanden or sanyo, or whatever it is, I will check to verify the brand. If you converted, you would have to have two hoses made to replace the manifold that connects to the R4 compressor.
I really went through a nightmare with an R4 on my wifes 300TD station wagon, and hate the R4, but a NEW one, not a rebuilt would probably work out okay.
Don't forget the ester oil, a new filter drier, and it wouldn't hurt to pump it down for 24 hours before charging it. The good news about changing to R134 is that if you have a leak you don't lose $100 or more worth of refirigerant.
Best of luck with it.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in