The premise is just as Bob's picture above shows - Because the oil is fed down to the mains through the same passage that connects through the camshaft to feed the rockers, the oil going up to the heads can take volume away from the #4 main, which in turn can limit the oil for the rods that main feeds. IMO - you need some really large clearances, a grooved #4 cam journal or grooved bearing, or really high sustained rpm to encounter that issue. What are the plans for this engine? What heads are you running?