The error code E4 is related to water flow, similar to the PL error.

It generally means that there is enough water flow to activate the flow sensor, but not enough to keep the heat exchanger at the required temperature (causing the pressure to rise on the manometer).

You should ensure that the flow is good, not obstructed, and that the inlet and outlet valves are open 100% with the bypass set at 50%.

If the issue persists, check if the error code occurs at startup or randomly.

If it happens randomly or only in the morning, it's likely a flow issue, such as improper filtration priming or an installation problem.

Is your heat pump at the same level or height as the filtration system? If not, the problem likely originates there, and you may need a check valve.

If the error code appears at the start of the heat pump, you should check the High-Pressure and Low-Pressure sensors.

By swapping these two sensors, verify if the error code changes. If it does, then the sensor that causes the change should be replaced.