4 bar is only 58.8 psi. factory is around 170 (best I remember) even if you had low compression pistons in it thats very low and doesn't fall into that "keep them all within 10% of each other" rule. Also yes they do all seem rather low. Cyl 4 is by far the worst and it the one I would focus on because there's definitely something wrong with it.

If a cap of oil in the cylinder doesnt make the compression come back up then yes headgasket, valves, warped head, cracked pistons, any of those could be the culprit.

if you want to find out more do what they call a "lead down" test. you basically set the piston to top dead center and make sure the valves are closed, put about 30-40 psi into the cylinder. listen for where it comes out of in the block or around the head gasket to try to pin point the issue.