Hello: You can stop flow through a tank with a three-way valve, but I’m not sure you can remove pressure from the tank. I’d want to be able to do both so servicing is easy. Two three-way valves will do it, but it’s always good to keep the plumbing from looking like it belongs in the engine room of a submarine 😀 Simple is simply harder than complex.
The four port valve hookup basically would attach between hot and cold lines into and out of the heater. In one position, you have normal operation. In the other position, cold flows straight across to feed the house and both heater lines are disconnected from service. Clear as mud?
Yours, Larry … (practiced in the art of opaque thinking)