What do the rest of the plugs look like? Do you care to post some pics of the plugs that are burning the cleanest? It may be temperature range, but if the other cylinders are right on target, then hotter plugs may not be too good in those cylinders. Maybe check resistance on those four plug wires to see if anything’s off compared to the rest of them. If the plugs are on the cold side, it may be showing those cylinders are going a little rich during the pull. I know that’s probably unlikely in a boosted application, but that is pretty odd.