Yeh, normally you do a standard compression test dry, then a wet one (adding a small amount of oil to seal the ring gaps, but that'll only tell you is you have ring or bore wear), but yeh what you have done is basically the same thing if you got the gear.
If you crack the injector line up at the injector with the engine running, like you were bleeding the system, does the 4th Cyl line seem to spray the same amount of diesel if you were the crack any other cylinder?
How hard is it to remove the line and blow compressed air through it to make sure its 100% cleared, diesel pressure from the pump should clear it, but not if its running/blocked by the top of the injector.
Basically make sure the fuel line to the 4th cylinder isnt blocked or damaged
If the line is OK, your looking at a pump problem.




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