John, your 300SEL doesn't have a MAS, so that ain't it!
Sorry, I don't know for sure what the code 4 is, but someone on here should reply.
On the spark plugs, it's prety important to use non-resistor plugs in the 103 engine. See the sticky at the top of the Tech column. Bad news is, these are virtually impossible to find these days.
DG
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