How do you know it's only running on 4 cylinders? I don't see a V8 running at all with only 4 cylinders...maybe 6 at worst. If you think that you have four dead cylinders, pull the plugs from the 'dead' cylinders and see if they are soaked in gas and look way different from what you have in the picture...if they look the same you don't have a dead cylinder.

That spark plug looks extremely lean (whitish color). If your engine is running really rough and popping through the carburetor, my money is on you have a vacuum leak.

Does the motor seem to want to idle really high and run really rough? If so, a vacuum leak would cause that along with that lean condition.

Holler back and let use know and we can help you check for the vacuum leak if that what it looks like it may be!