Well, I took my car to a dealer to have the codes read. They told me a Misfire code was what had triggered the Check Engine light. They could not replicate my problem, but told me that the third cylinder's spark plug wire looked damaged. They suggested that I replace the spark plug wires and spark plugs. This got rid of the problem. My only question now is: why was the car usable under 4000 rpm, if the problem was in the spark plug wires? This confuses me. Thanks again to those who responded, and to anybody who responds to this message!