That's a long post... lol What I got from it is that your cylinder 4 is misfiring? Is that all it's doing? (Don't feel like reading all of it again... haha sorry) Did you gap your plugs correctly? Are your plug wires firmly clipped on to your plugs and coil pack? What do your spark plugs look like? If you unplug spark plug 4 does your idle change or does the car run the same way? Are you sure number 4 is working on your coil pack correctly?
Just some things to think about and inspect, and always double check everything.
I'm thinking since I think you said you changed your coil pack like twice.... I think that's what you said in all of that rambling. jk lol It could possibly be in the plug/wiring harness that connects to the coil pack and back to wherever it comes from. (I'm not an electrician, so I have no idea about all of that stuff.) But I'm thinking if it's not your plugs or wires, or your coil pack... it has to be something in the wiring to the coil pack or ignition somewhere. (maybe a mouse got in there and chewed something it shouldn't have, lol you never know) You said your injectors are okay so I wouldn't think it could be that.
But since it's cylinder 4, I would start at the cylinder and work my way out exhausting all of the possible causes. And i'd start with the cheapest solution first. Don't want to spend too much money.
Just thought I'd throw my 2 cents in there. I'm sure you have already tried all of this stuff I've told you... but it never hurts to double and triple check. Hope you figure it out soon.