Have you verified that there is no spark from that plug wire for cylinder #4, if so you most likely have a bad coil pack, where the plug wire plugs into. The coil also sparks the #1 cylinder and they're common to fail on one side. You can pull the plug wire off the coil and hold a grounded test lamp approx. 1/2" from the tower and check for spark to jump from the coil tower to the tip of the test lamp. If no spark here this is your problem. There's always a possibility the ignition module is causing the coil not to fire, the module is what the coil sit's on, but this is less common. One test you can do before replacing the coil is to swap the two coils around and see if the spark is lost on cyl #2 or #3 rather than #4, if it still is #4 than the module is at fault. If you have good spark all around, you may have an injector issue, or injector triggering from the computer, or even a loss of engine compression.

-Rip