Question
We hear a loud bang (a single one) ONLY when the automatic sprinkler system turns off and only on the last valve (system has four valves).  Could the last valve be bad and that is what is causing the bang?.

Can you guys help by suggesting some solutions?

Answer
Hey Mike, what you are experiencing is a water "hammer". Briefly what is happening is that as all the other valves are running, one is turning on as the others are turning off, and then the last one shuts off and all the water stops or "hammers" the system. It is probably nothing to  do with this particular valve. You can install a "water hammer arrestor" on the system. Basically what this is, is a cylinder with a spring in it, that absorbs the shock when the system shuts off. Good luck and have a great day.