Code 4 is air injection inoperative. The next step is to read the codes on the HFM which is pin 8. You need to build the code reader.

somewhere in these forums, a very clever MB mechanic explains the BITE diagnostic system, the steps to find the "true" code. the blinks from the built in button only lead to another set of pins which blink with reader but that also is only a general area of fault but closer to true fault and fix. Keep searching the forums and build the code reader, I did and it works great. Good luck, FYI if car runs fine some people disconnect the CEL.

I've got mostly everything to build the code reader just need to get some wire.

The light went out finally after I reset the codes. But my fuel economy just seriously tanked.

example, with my normal driving and occasional AC use the first bit of the fuel guage from full to the next hash mark is around 90 miles.. then to half a tank the total is around 180-185 miles. Today... 117 miles. I'm waiting to see how the full tank goes. I'm going to double check tire pressure tomorrow

I think you're wrong. According to this thread (E320 16 PIN DTC Charts inc. pin 8, pin 14) "4 Air injection system faulty hot film mass air flow sensor with hot wire from the" diagnostic module(led button deal). But I do need to build the code reader to check pin 8

I know the air pump works because it turns on when it should

I miss owning a diesel.. none of this electronic crap

The factory shows it as just "air injection inoperative" - which doesn't necessary mean the air pump is bad. Typically the DM only scratches the surface - again you need to read the other codes as the factory suggests. The Baum book is great but it has quite a few mistakes - for example "hot film mass air flow sensor with hot wire" is complete gibberish. Hot film sensors don't have a hot wire.

http://www.startekinfo.com/manual/volumes/e3/8_4/m11.jsp