Given that personnel matters affecting an individual on the board often have mandatory recusal of the affected party, that's likely a 4-0 or two 4-1 decisions, depending on how they were structured.

A single 4-0 decision would imply the bylaws allow any group of members to oust all the other members by making a single proposal to oust everyone but their group, thus automatically forcing everyone else to recuse themselves :p

Yes, this can happen, though generally the ousting party would also need to be able to point to some policy or legal transgression to do it without putting themselves at risk of a lawsuit.