Thanks for trying to help.
I'm sure someone in this community knows the problem and the solution.

Well known problem the carbon deposits.
But a few weeks ago I had to mount a new cylinder head gasket because of a
crack in the gasket causing a coolant leak.
An inspection of the cylinder head revealed nothing strange.
No deposits on the valves or in the combustion chamber.
Cylinder no 4 had traces of oil on the pistion top. Not much, but using a
paper towel I got small traces of oil. The other cylinders was all fine.
The oil consumption is normal. Close to nothing.

The car runs well, and has enough power.
The problem started more than a year ago, so I'm starting getting a bit
xxxxx (fill in with suitable word...) now.

Ask the Volvo shop... no chance... They won't answer unless they get paid
for it...
Once I tried, but the answer was: spark plugs, cables, water in the fuel,
and so on...
Nothing revolusionary new...