Coolant in my spark plug

My engine has been having a rough idle due to misfire at cyl#4. I changed the spark plug out and it fixed it. It came back again today and I found trace amount of coolant in my spark plug that was causing the misfire. Did anyone have this problem before?

Head gasket?????

Thats what I thought but my radiator fluid is nice and transparent green not milky. And the fluid was in the sparkplug hole not in the combustion chamber its really weird how it got there.