Cylinder missfire

A year ago my 2006 Prius started running a little rough. So I replaced the spark plugs. Which it didn't change much. So about 2 years later, I got a cylinder 4 misfire. I learned that I didn't put OEM plugs in, so, I put OEM plugs in them. The error code went away and then popped up again about a week later. and it goes through phases where one day the check engine light will be on for about a week then it will be off for about a week. and on for a few days... and then off the next week(or so). each time I checked the code I got a cylinder 4 misfire.

Then I thought it must be the coils. So I swaped the coils, I still get the same message & check engine light in the same manor.

Not sure what to do next? Is the a EGR valve or the cables? what should I do?

 

Thank you both for replying.

My odometer is roughly 320,000 miles on it.

I forgot about the injectors! that could be it.

Now, Last year and two before that, I randomly lost all oil. No leaks, or puddles to be found. Both times I replaced the oil and it maintained the oil in it without any other issues or disappearing oil. I had a mechanic look at it and no one could figure what was gong on.