[H. L. Chauvin]

From experience ... very possible scenario:

No. 4 cylinder rings sticking in the closed position ...... very common vintage engine problem after lots of carbon build up.

Bottom of cylinder after engine use always has a smaller diameter .... & top of cylinder has larger diameter .... one can "see" this at the ridge.

Piston goes to smaller diameter at bottom of cylinder where rings compress and get stuck with carbon/oil crud in the ring grooves ..... when piston goes to the larger diameter top, sticking rings rings do not have enough mili-seconds to expand ..... hence, allow blow by & more carbon on plugs.

Ever consider how fast the piston is traveling up & down and how often ring expand & contract on every revolution.

Years ago, many poor who had more time than money used to clean rings and ring grooves and re-install same to prevent harmful plug fouling & loss of power.