I removed a 5th council member through voting, there were 4 left and I saw a message that there needs to be at least 5 of them within 48 hours or guild will be disbanded.

3 out of the 4 remaining council members have voted to replace the 5th member. But the vote has not passed. Does the vote need to be 4/4 to make a new council member? Or is it bugged?

If there are less than 5 votes possible then there needs to be 100% unanimous vote, so yeah 4/4 need to vote.