Just a little additional info....
The three A/P bars light up at APP selection but there is still only ONE A/P operating as evidenced by the PFD FMA of CMD. That changes to LAND 3 at some point AFTER 1500' RA - the interval being the requirement for the self-check confidence test.
The elec bus isolation occurs so that, in the event of any single engaged bus failing, the #4 bus will take over the failed bus via the Sync bus and the failed gennie's BTB.
Apologies, if that's "teaching granny to suck eggs" etc.