Fouled spark plug in one cylinder (only)


Beginning a year ago, began seeing an increase in oil consumption:  Then suddenly, severe misfiring.  Inspection revealed that spark plug B in cylinder 4 was very heavily crusted/covered in Oil. Replaced plug and all wes fine... for ~ 4,000 km.  Then again, after only about 4,000 km, problem reappeared. Changed plug again, but this time, plug was dreadfully encrusted with oil/carbon.  (Spark plug A - in the same cylinder, gets much less so). 

To me, it seems that this suggests oil leak is from valve or something related to valve, (guides??)  rather than cylinders, rings, etc. Does this make sense? If so, is it recommended to first verify by doing a compression test to rule out the cylinder block, rings etc? And if yes, short of removing the heads, any way to verify the likely culprit? (  The car is excellent in most other ways, so any thoughts or strategies would be appreciated, BEFORE I go down the wrong road)!    With thanks