One guard suffices to cover a 4-gon, no point interior to the polygon remains uncovered by the guard.
All 5-gons are completely covered by one vertex guard. There is not a single point interior to the polygon which is not covered by the guard.
RESULT: It is not possible to construct a polygon with less than six vertices such that the guards see every point of the boundary of P, but there is at lease one point interior to P not seen by any guard.