I have found two other points of square by this formula
 xc = (x1 + x3) / 2.0;
 yc = (y1 + y3) / 2.0;
 dx = (x3 - x1) / 2.0;
 dy = (y3 - y1) / 2.0;
 x2 = xc - dy;
 y2 = yc + dx;
 x4 = xc + dy;
 y4 = yc - dx;
But i could not understood why this works.

Then I use vector product to determine position of point etc.

Can someone explain me ?