For #4 inop, the #4 load bus will be powered by #3 (if I recall correctly).
My engineering manuals concur with this. Gen 3 will be providing power to the sync bus and Bus 4 during A/L.
If another generator fails above "alert height" (200' RA), bus isolation will be cancelled and LAND 2 will be annunciated. With Bus Isolation removed, all busses will be powered (i.e. all computers/sensors will still be powered). You will only lose your power source "triple redundancy".