Try to block your return line and see what happens. A Holley blue pump can be run without a return line. I would raise the fuel pressure to at least 6 PSI at idle, 4 PSI is too low for most carburetors, especially a Holley Carb. It seems that either you have a massive amount of fuel returning to the tank and not letting the pump build pressure, your pump is getting weak, or the regulator is not operating correctly (which can cause the massive fuel return to the tank). If you shut of the return line and the pressure goes up, then your pump should be fine and your regulator may be the culprit. If you shut off the return and you still have low pressure I would start looking into the pump itself, or maybe see if there is some blockage in the tank pickup itself. The regulator could have some debris in it that is not allowing it to work correctly, Holley regulators are easy to open and check inside them, the pump is also easy to open on the bottom to make sure you have no debris inside of it. There is a small screen on the inlet side of the pump that I have seen full of junk and can starve the pump. Holley blue/red pump are pushers, they need to be under the fuel level so that fuel basically gets to them by gravity and they just push it on forward, they are very poor at sucking fuel from a fuel tank, they really need to be gravity feed. Also check all the hose connections in between the fuel pump and the pickup, a hose that is a little loose can let in air from the outside and cause a low fuel pressure issue. If everything in the regulator, hoses, pick up etc.. check out, then your pump is probably weak. They have brushes that do eventually go bad, and they can be replaced. Also make sure you have 12 volts going to the pump at all times.
