I had four wires - yellow, green, red, and white. I moved the white wire
from O/B to W1 and that was what fixed it. There are a bunch of websites
out there explaining what each wire does, though it's painful to try to
figure it all out. Good luck!
Update for those who also find this thread:- I contacted Google Nest
customer support directly and, after we tested that the cooling system
worked fine but the heating did not, the tech suggested that I switch
the O/B wire to the W1 input because he ...
I am also having this exact issue. Installed new Nest on a heat pump
that was working fine with the old thermostat. Now heating continuously
and can’t get it to stop. Trying soft and hard/factory resets.