There are a ton of threads, search P0304. This misfire code can be everything from needing plugs replaced, to your motor having ringland issues.
Start diagnosing with the easy stuff, plugs and wires, moving them to different cylinders and check lights.
Best thing to do is a compression/leakdown test if you have the equipment. It's pretty cheap to get done.
You can search a ton, but there are so many causes you need to get your car looked at professionally if you don't have experience working on cars.
Originally Posted by pghana66
have u had the timing belt serviced?
How would this be the first thing you ask about for a misfire?
Also, P0304 is a cylinder 4 misfire, not 3.