What is wrong with my car?

I have a 2000 honda crv. The check engine light came on about a month ago and the car started shaking when idling and stopping. We drove to the auto parts store and they scanned it and came up with codes po300-po304 which is random misfire of cylinders and misfire of all cylinders. We replaced the plugs, plug wires, distributer cap, rotor, ignition coil, fuel pump, and air filter. It still does exactly the same thing and we dont know whats wrong. The guys at the store said they dont know anything else it could be. We put some fuel injector cleaner in today so maybe that will work. We are also going to adjust the valves once the engine cools down. I need to know what else it could be?