There was at least one recent instance here where a reading of that low was caused by maladjusted valves. I'd double check the clearances on that cylinder before proceeding further. Were there any symptoms leading up to the adjustment and compression check? How long since the last valve adjustment. A too tight exhaust valve could have been burned. What was the initial clearance on #4 exhaust before you adjusted it?

It's a DIY as far as removing the head and reinstalling. Valve job itself will involve a machine shop. I don't think you should be jumping to the valve job conclusion just yet.

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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