Check the spark plug, looking at the tip. The color and build up will help show if you are getting proper spark. Engine compression could be down as well, check the compression.
Random misfire, cylinder 2 misfire, and cylinder 3 misfire. see related question below.
What causes misfire 302, 304 and 306
Bad plug Bad coil Bad plug wire Bad valve Blown piston
Code P0304 on 2000 Ford Ranger is: Misfire at cylinder 4 detected. May be cause by a bad spark plug.
Cross-threaded spark plug.
random/multiple cylinder misfire!
The exact cost to fix an engine cylinder misfire on a 2000 Mercury Mystique will depend on the exact type of engine it has, but can cost around $1,000.
Cylinder 1 Misfire Detected
Cylinder Number 1 Misfire Detected
Code P0304: Misfire at cylinder 4 detected.
Cylinder #2 Misfire
A P1399 code is a cylinder 1 misfire code