There is no requirement for FCs to maintain 4 account accounts to purchase or retain housing.
There is only a requirement for FCs to have 4 members to purchase housing. None of them need to be active other than the player that is going to make the purchase.
There are loopholes within loopholes in the FC system that allow a single player to abuse the system to get more than is intended after a little initial help from other players to get the FC registered.