Oil in cylinder 4
 

Alright so this last weekend I decided to do water pump, tensioner, pulley and timing belt on my 2006 Volvo S40 5 speed manual. After I got it all done I decided to do the spark plugs as well. First 3 spark plugs were in good condition and showed excellent wear on the actual post of the plug. Got to the 4th cylinder/plug and there was a little oil on the top of the plug but not fully submerged or anything. When removing the plug there was oil about 3/4 down the thread but it didn't look like there was any oil on the post and the actual post of the plug was wearing normally like the other plugs. Any idea where this oil is coming from? Seen how it could be oil cap but that's pretty far away from cylinder 4. Also heard pcv could be It, but if so why only cylinder 4? The fifth spark plug had no oil in or around it and was in pretty good condition as well. I've done valve cover gaskets on other cars, but looking into this one seems really intricate and may not even fix it. I did read there is o-rings around the plugs and that could have been the fail point and would make sense why only one is leaking. If anyone has experience or anything helpful let me know. Thank you.