Code P0304 only comes after idling

Earlier this week my truck was throwing me a P0304 code. I assumed it was the spark plugs so I changed all of them. Reset it and it gave me the same code. Then replaced the coil pack for the fourth cylinder. Now it only throws the code if I’m idling longer than a few minutes. When it’s idling you can feel it shaking pretty good. I’ve looked at a couple of other forums and haven’t found any solid answers. Any ideas?