Rough idle

I have a 2001 Ford Taurus SES, 103000 miles with the 3.0 single overhead cam V6. It started to run rough on idle, then will occationally lug down while being driven at just about any speed. The check engine light came on and the the code reader came up with 0304 a misfire on cylinder 4. I recently replaced the plug wires and replaced the plugs last week, but no change. Any help on what it could be would be great. Do I need to replace the ingnition coil or could it be soemthing else. Thanks in advance.