Toyota Repair: 1999 Sienna Code 304, cylinder misfire, coil pack

Question
My 1999 Toyota Sienna's check engine light keeps coming on. The code is 304, #4 cylinder misfire.
From diagrams, #4 is the center front cylinder.
I pulled the coil pack and spark plug from cylinder #4 and #6, cleaned all the connections and the plugs. I swapped the plugs and coil packs between the 2 cylinders and reset the code.
Took the van for a ride and things seemed OK, no code and the engine was running smooth. The next morning on first start up, the engine was running rough and the check engine light came on, code 304. What else can I look at? The wires don't seem easily replaceable.
Any help would be greatly appreciated!
Thanks,
Chuck

Answer
Since after switching the coils the misfire is still on #4 we can assume that the coil or spark plug are not the problem, Toyota has issued a service bulletin regarding this condition, it involves replacing the fuel injectors, if you'd like to have a copy email me directly at tedrttr@yahoo.com/