I believe this ultimately depends on what type of cam you're using. If you're using a cam that has fully grooved bearing races, then I believe the mod would help. #4 main also feeds the cam, then the rocker shafts. If it's oiling the rockers full time because of the cam, then the pressure drop is going to be higher at #4 because it has a bigger bleed vs #3. #2 is the same way, but apparently doesn't have issues because the rate of flow at the oil galley is significantly less. Remember, "An object in motion tends to stay in motion."
This may explain why some have issues and others don't. They may have different cams, or restrictors going to the heads. IMO, just drill the oil galley to the mains to 5/16"
The pressure of a fluid in a system will always drop at different areas in the system. The oil filter sees higher pressure than the drivers rocker shaft. Same goes for the right/left oil galleys and the rocker shafts. Might not be a lot of pressure drop, but it's there.
