I agree with you on the Intake valve not sealing, for sure the pressure in the manifold is increasing as the piston in the offending cylinder is coming up.
The same piston is creating less crankcase pressure, quite likely because it has less to push past the rings in the first place.
Relative to the strongest cylinder your other 3 are at 88, 80 & 68%. You do have an issue in at least 3 maybe all 4 cylinders
I think you need to make that adaptor, pull the injectors or glow plugs and see what you have physically measured in the strongest cylinder. But you already know that