Thank you. The wires and everything seem to be in good shape. It’s throwing code P0304 now. Just that code.
The runner still needs to be checked that it is opening and closing. I had the exact same codes on my 2016 Mustang. The Mustang had a vacuum actuated Manifold runner and everything appeared fine until I removed the intake manifold. It turns out that the actuating rod had failed, but the vacuum actuator was working normally, it just didn't connect to the runner.
I do not recommend pulling the intake at this point. The number 4 cylinder is the second one from the front on the driver's side, so you only need to focus on that runner and cylinder. The runner should open and close based on throttle position. You need to figure out a way to check this out. Since the motor needs to be running to actuate the runner, this could be difficult.
If it were me I would first do a compression check on number 4. You've already done the plugs, COP and injector. A cheap endoscope that attaches to your phone would allow for a look see inside the cylinder and perhaps inside the intake and the runner. There is a LOT of chatter about the P0304 code on the forums, but I didn't see any real solutions. Good luck and keep me updated! BTW, pulling the intake on the Mustang was a piece of cake.