Misfire detected and injector circuit open

I have a 2000 Buick Century, 3.1 V/6, air and auto. 111,000 miles. For the last couple of months the service engine soon light has flashed. It has been known to be off for as long as a week but it normally flashes. I have a code checker and normally it says PO304 which is a "misfire detected in number 4 cylinder". For the first time today it also had a PO204 which is "Injector circuit open number 4 cylinder". I have changed all the spark plugs and wires but now this injector circuit code came up. Any ideas out there?

Check the connector at that injector, remove the connector look for corrosion in the connector clean up and reinstall, also if possible, check for injector pulse before and after you do this, use a noid lite, if no pulse when done, you will need to trace the wire back to the pcm.

I have a 2000 Buick Century, 3.1 V/6, air and auto. 111,000 miles. For the last couple of months the service engine soon light has flashed. It has been known to be off for as long as a week but it normally flashes. I have a code checker and normally it says PO304 which is a "misfire detected in number 4 cylinder". For the first time today it also had a PO204 which is "Injector circuit open number 4 cylinder". I have changed all the spark plugs and wires but now this injector circuit code came up. Any ideas out there?

I have never done either of the things you suggested. This is my first car that had injectors. Is a noid light a continuity checker? I also don't know what a pcm is. I have worked on a lot of cars but this is my first with injectors. I am old school with points and rotors.

Oh boy, this is trouble, yes a noid lite is a continuity checker, when plugged into the injector connector it should flash as the engine runs, pcm, is the computer that contols all these functions. A subscriptoion to Alldata.com for you car will walk you through all these steps, but you may need on site help with this.