Engine Misfire And Carbon Fouling Plug On Cyl. 4

Are you sure they're carbon fouling? I've run synthetic oil for a number of years and I find I get dry deposits more often than oily residue on the plugs so it may be oil unless you're certain. 4 and 8 are at the back of the engine so the tilt and oil drain-back can cause some oil to run down worn valve seals, especially if there's some issue with the drain path. I've also had oil get into cylinders across from one another when an intake manifold gasket failed (though not on a 351w), but checking the intake torque never hurts. Could also have a problem with an oil control ring, again if it's oil.

Do you get smoke out the tail-pipe at start-up, or under other scenarios? I imagine you're using aftermarket fuel rails, is it possible their design is causing more fuel pressure at the back-side of the rail (if your fuel pressure is maybe low or uneven and the computer is compensating for the leaner cylinders causing richness in the back two). Just a complete guess but if it's been doing this ever since you installed the intake / fuel rails it may be worth considering.

If your leakdown test and compression test were good, I'm not sure what you'll find by pulling the head.