A figure formed by joining four points in an order is called a quadrilateral.
Consider the analogy of four people as four points.
In the first case, there are four collinear points which cannot form a quadrilateral.
In the second case, there are three collinear points which can form a triangle, but not a quadrilateral.
In the third case, the two pair of people face each other and thus form a quadrilateral.
In the fourth case, the two pair of people face exactly opposite but hold each others hands from behind and side and thus can for a quadrilateral.