Unanimous written consent

On a 5 member board I think it is understood that unanimous written consent must be of all 5 board members. So if a board makes decisions outside of an open meeting it would require the written consent of all 5.

But what if a board member resigns in mid term, leaving only 4 board members. Can the 4 member board now legally make decisions outside of an open meeting with the written consent of just the 4 board members?

As Kerry said, you now have a Board of 4 vs. 5.

A unanimous written consent would be all the seated Directors.

If the number of Directors do not constitute a quorum, then unanimous consent can still be used but only to appoint someone to the vacant seats.