Error 4 on the Dr. Morepen BP One Blood Pressure Monitor (BP-14) usually indicates a problem with cuff fit or pressure detection during measurement. Check that the cuff is wrapped correctly around the upper arm, positioned at heart level, and neither too loose nor too tight. Ensure the air tube is firmly connected to both the cuff and device, with no bends or leakage. Sit still and avoid talking during measurement, and try again after resting for 5 minutes. If the error persists, replace the batteries and retry. If it continues after these steps, the device may need servicing or replacement; refer to the user manual or contact customer support.