I am solving the same "E4 error". A litre countre valve is placed on start of water pipe. Closer to water input. Activated by AC230V, it works like on-off switch. One or two coils.
Preasure of water is detected by sensor, placed on other end of water pipe. Closer do engine. Status of this one is dependent on water preasure. It means if valve is closed, this sensor have to detect zero preasure. If valve is open, this sensor have to detect normal water preasure.
Information is compared and presented by electronic board with display.
Honza
November 2012