P0304

This is my first post on this site so let me know if I am asking too much at once or any other issues my post may have. OK so I just bought a 2000 Taurus SE with a 6 cylinder 24 valve DOHC motor. I knew it had issues. My code reader tells me P0304 is the reason it is running bad. It's been a while since I have turned a wrench so my learning curve is straight up.I tried a few basic things like swapping the plugs and the coil pack on top and the boot from number 5, which had no effect. Which was a good thing since I found that the #5 plug was loose and the boot was all carbonated up? Oddly enough the #4 plug looked like it had carbon build up due to not firing even though my tests indicated it is getting spark. I only have a Haynes manual from VIP so I may refer to an item incorrectly until I get actual shop manuals. I also put in high test and ran a bottle of fuel system cleaner and a bottle of injector cleaner.I held the gas to 3k rpms while in park the light went out. As soon as I let up it came back on. I replaced the fuel filter and front 3 plugs just as a quick try. Still ng. When I get it rolling the light goes out. I can actually feel it get better some times. I wil be doing 30 or 40 and feel it get a boost and then the light goes out. If i come to a stop it is back on. Start drive at at least 1500 to 2000 rpms and the light goes out. Today I pulled the mainfold to change the plugs and swap the injectors to see if it fixes or changes the code. No Change. In the process I soaked the throttle body and all the other intake parts with carb cleaner. I must say it looks good runs like crap.I replaced all o-rings on the injectors and swapped them around just in case the #4 was bad since the code is P0304. I replaced the air filter.I also had to replace the coolant hose that is closest to the air box since in the process of replacing the PCV I broke off the little piece that sticks up. I swapped around the coils. Took it to a shop to get a sticker and they called saying it was running so rough they couldn't even do any further testing to pass inspection. They said number 4 had 40lbs of compression. I pulled the valve cover and as far as I can tell with it running or not the valves seem to be working. No broken springs or obvious issues. Now my low coolant light is on. can this be due to the fact I only put water back in just in case I had to drain it again? Or could i have messed up a wire to a sensor? I think there are at least one or two in that area of the PCV that I replaced. I haven't been able to get my own compression tester yet but that is my next plan to verify what they said. If it is correct and the valves seem to be moving should I shoot some oil in the plug hole and do a dead crank test to see if it comes up? Thanks for any and all help.