Carbon build-up on Cylinder #4

[justjuiceit4]

I have a KA24DE and the other day I was changing out my sparkplugs. They all had a nice tan color to them. I decided to check out to see how my pistons were. #1,2 and 3 were all perfectly clean, but #4 had some carbon build-up. I checked it with my compression tester and sure enough it had more compression.

Now the plugs gave no indication of oil fouling and it did not look like any oil was leaking in there. However, I did notice the last valve cover bolt was just sitting there and the top head had actually snapped.

What do you think caused this carbon build-up? Is seafoam the best way to get rid of it without pulling the head?