Misfire on cyls 1-4 but runs fine??

[xlr8ing]

Just to update, so far so good. No flashing engine light, and the readiness monitors are all good after the O2 sensor swap.

[xlr8ing]

Another update...a few says after my last post, the flashing engine light came back. As usual the car still ran perfectly.
So figured I'd try another set of spark plugs, though my last set of plugs were only put in at the end of January and were Volvo OEM plugs. Not sure how many km, but estimating 12000-15000.
Within a day the light stopped flashing without me having to reset the ECM.
While the plugs were out I looked down to see if I could see the piston tops. They look a like they have some carbon build up, but not terrible.
I figure I'll do a seafoam treatment once I have some miles on these plugs see if that helps at all.