Hi,
Code 4: "Air injection system faulty hot film mass air flow sensor with hot wire"
Unfortunately, I think that's bad news and probably de air mass meter is starting to fail and that's expensive
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=26H19PM5K26H19PPRQ&year=1994&make=MB&model=E-420-001&category=B&part=Air+Mass+Meter
I've read you can clean it using some product (electronic contact cleaner) but I have never tried it. Don't touch the film inside the meter. Is the car running ok? You could try disconecting de meter with the engine running and observe for changes.
Did you delete the code to check if it comes back?
I hope this helps.