What should I do if my heat pump displays an E4 error code?

The E4 error indicates compressor air discharge high-temperature protection. Confirm the outdoor unit specifics and specific model of the compressor in question before troubleshooting (Page 29). Restart the system to check discharge temperature sensors, examine electronic expansion valve connections, confirm indoor/outdoor unit capacity match-up, inspect for refrigerant leaks/blockages, adhering to best practices provided in the manual.