P0304 has me confused?

[3500srw]

First a little background: I bought the car in March this year. It had 4 DTCs EGR and mis-firing #4 cylinder codes. I tried to clean the Egr but ended up buying a new one. I put on new plug wires and new plugs. I carefully cleaned the Ignition module and coil packs used corrosion preventive dressing and all. All codes were gone and all has been well for the past 2 months. I've even used a couple bottles of Chevron Techron fuel cleaner in that time and have logged 7000 miles. The last bottle was about 3 tanks ago. The car has been getting 40-42 mpg highway. All was fine until today!!

I started the car and immediately noticed unusual vibs coming from the engine. I got out the drive way and drove a little bit and turned around and parked it. P0304 is the only code present which is "mis-firing of #4 cylinder" I took the ignition coil packs apart from the ign. module again and cleaned it again. I have lightly cleaned the throttle body(sprayed a 1/8 of a can while it was running). I switched the coil packs to see if the location of the mis-fire would change, nada! I check for spark and it has spark. Anyone have any ideas?

Thanks in advance