What could cause two of my cylinders to be stop working?

I have a 1992 240sx kad24de, cylinders 2 and 4 are not firing. So far i have changed the fuel injectors, fuel filter, brushed the spark plugs and checked gaps, adjusted the distributor and replaced valve cover gasket. I do believe there is a broken gasket or the cylinder head is crack but im not sure which; i have added stop leak by enviortek, which did not work. When i take the spark plugs out after 10 minutes of running they are black which means its running rich. Also they feel like they have alittle engine oil on them. Could this all be caused by the gasket/cylinder head?