Contrary to what was posted earlier, I don't believe that switching a bad plug to the #1 position will help. With series plugs, the current has to run thru all plugs to ground. The ground goes from number one to the block. If #1 is burned out, the circuit cannot be completed to ground and none of the plugs will work.
If the relay is bad, you can jump from the positive post on the battery to #4 plug. This will energize all the plugs if they are good and you can start it.

I'm not sure what PEH means. I don't know how you can tell a good plug from a bad plug. I think that if there is a bad plug in a series system, you can bypass it by jumping from the heavy wire coming in to the heavy wire going out and complete the circuit that way, energizing the rest of the functioning plugs.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13