Do you know whether the points are in order? (a,b) to (c,d) might be a diagonal?
Do you know whether the points are, in fact, a rectangle?
If you know that it's a rectangle, but don't know what order the points are in, you could find three lengths from (a,b) to the other three points
{hint: length = sqrt((a-c)^2+(b-d)^2)}
and then the shortest is the breadth, the middle is the length and the longest is the diagonal.
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