Disconnect the number 8 wire from the controller and turn on 4. Are the both still running? If not there's a problem with the controller. But I'm assuming there are bare wires touching each other. Look at the valves or where any wires are exposed. Do any digging lately?
Sometimes the gardeners will hit the wiring with a weed eater and hook them up wrong.
Well a month or two ago I did have some irrigation techs come over to price me to lay a new cable down from my controller to the PSR. As the PSR was not receiving signal (24v) from controller to turn on, I fixed that issue myself.
However the techs took it upon themselves to diagnose the problem, with that they dug out one cable outside the yard near the controller box, and cut about a 8" off it, then used another wire in between the missing part to re-attach the cable together with screw caps. I changed the screw caps to blue ones as they had used regular orange ones.
What I did notice in the controller box was that zone 4 wire is black and zone 8 was dark blue, could it be possible that joining the wrong colors caused this?