@Jim - with the master valve manually open and disconnecting the blue wire from the Rachio, the Rachio should be able to run all the zones.
Upon further review of the wiring, it looks like the prior setup had a wired rain sensor (two black wires) and a remote control unit (blue, red and white wires).
Rachio won’t support/use the remote control unit, so the small red and white wires in the 24 VAC - and + can be removed. I see the small blue wire doesn’t appear to be connected. (If that does go to a remote control unit).
The wired rain sensor should have one wire in S1 (or S2 - your choice) and the other wire in 24 VAC -.
As far as finding the fifth valve - do you have a metal detector? I’d try having the Rachio run the fifth zone and see where it is watering. Then I’d see where the nearest known valve is and walk from the nearest known valve via right angle path toward the fifth zone area to see if I could find it. One may need to call out an irrigation specialist to find it if necessary. If the Rachio runs the fifth zone with the master valve open, the it may not be necessary.
No resistivity between the thick blue wire (master valve) and the thick white wire means there’s not a circuit there. Not sure why that would cause a master valve zone fault. I might double check/redo the connection at the master valve. Use water tite wire nuts for that connection.