Why won't 2003 Honda Civic SI go over 4000 rpm?

If a 2003 Honda Civic SI won't go over 4000 RPM, it could be due to several issues, such as a malfunctioning throttle position sensor, a clogged fuel filter, or problems with the ignition system. Additionally, a faulty MAF (mass air flow) sensor or issues with the VTEC system could also restrict engine performance. It's advisable to check for any diagnostic trouble codes (DTCs) and conduct a thorough inspection to identify the root cause.