@slope - was the voltage test done with the wire to zone 4 connected to the Rachio or disconnected?
Trying to narrow down the possible error domains. Give the above information:
Rachio servers and controller smarts - not an issue as it received the instruction from the web site and the correct zone light is blinking.
Water to the irrigation system - not an issue as when the valve is manually opened water flows out of the zone.
Zone 4 was selected for testing, have any other zones been tested? Is this a problem with every zone or just one?
My favorite question - is there a wired rain sensor in-line on the common wire?
Can an ohm test be performed on wires disconnected from the Rachio - zone 4 and common? Resistivity should be in the 25 - 30 ohms range. Higher resistivity could indicate a broken wire or a bad connection?
To rule out zone 4 terminal on Rachio, one could temporarily swap the wires on two zones and see if one or both zones work in their new location (e.g. 3-> 4 and 4->3).