The first two easiest troubleshooting suggestions I can offer are
1) Swap the wires between zones 3 & 4. If the valve with the problem moves with this swap, the problem is in the controller.
2) If you have an ohm meter, disconnect the wire for zone 4. Use the ohm meter between the common wire and the wire for zone 4. If the wire is good, the solenoid in tacked, and no bad splices, you should get a reading on the order of 100 ohms (i.e. more than 10 ohms, less than 1,000 ohms). Higher readings (like 10,000 ohms or 100,000 ohms) might imply a bad splice (perhaps the connection to the valve wires is bad). An infinite reading means a broken wire.
