P0304 code - what is the most common problem?

Before doing what you suggested, the vehicle having sat in the drive way all night, I thought I should turn it on and get it warmed up before I started switching the spark plugs. The truck started up and ran without misfiring. I got it up to normal driving temperature and turned it off and scanned it with my cheap Autel Scanner. I got two pending codes: P0300 and P 1130. I erased them and started up the truck again. The truck was now misfiring badly. I drove the truck for about half a mile and turned it off again. I scanned it again and got a P0304 code. I did the same thing the following day and the truck behaved the same as the day before. It runs OK after the first start of the day, but if I turn it off and start it again after it has warmed up, it starts misfiring badly. I now believe the plugs and coil packs are OK, since it runs normal after the first start of the day and it might just be the O2 Sensor. Could you inform as to the best way to test the O2 Sensor (the one in front of the catalytic converter)?

Before wasting your money on an item that WILL NOT cause a single cylinder misfire , Might I suggest once more , that you swap coils (#1 & #4 , in this case), and again try your series of starts and watching what code(s) return. .........OR buy the O2 sensor and replace it and post results. Think about it...how many cylinders is the O2 monitoring vs. how many cylinders is the coil on #4 affecting?...Just sayin'...let us know what happens and either way ,have fun...if I'm wrong , call me an idiot , if I'm right still call me an idiot , just because of how I'm phrasing this stuff!!!................

I put in a new plug in #4 cylinder and switched coil packs #1 & #4. I still get a P0304 P Code and the engine still misfires.

Possible valve train problem ,or injector problem. Monitoring 'live data' would be extremely helpful for this type of issue. Watch MAP sensor and fuel trim.

Had to break down and take it to a mechanic. Charged me an arm and a leg. Replaced two injectors, cleaned the others and the fuel rail. Fixed the problem. Had rusty deposits. I guess all gasoline nowadays has a percentage of ethanol that eventually causes rust that destroys the fuel system.