I'm not sure I would characterize needing some refrigerant every 4 years as a "leak". May not even be detectable. Auto AC systems are inherently leaky to some extent. Ok, this is not ideal, but is it really at the point of repair? Do a cost/benefit analysis.
If it were me, I would topoff the system and add some UV dye.
That way you would likely be able to see where the [very] slow leak is over time.
You can get a really nice Tracerline UV dye kit from thetoolwarehouse.net for around $55.
Best of luck.
1986 300E 5-Speed 240k mi.