You can use the sqrt function. So if x and y are 4 element arrays, then do get the diagonal distances, use the appropriate indexes, for example
diagonalDistance1 = sqrt((x(1)-x(3))^2 + (y(1)-y(3))^2);
diagonalDistance2 = sqrt((x(2)-x(4))^2 + (y(2)-y(4))^2);
You forgot to provide your image (so read this), but assuming you have a white square box on a black background, you can use regionprops to find the centroid of the box, then use bwboundaries to find all the (x,y) coordinates of the perimeter. Then use sqrt() to find the distances of every point to the centroid, and use findpeaks() (in the Signal Processing Toolbox) to find the 4 peaks, which should be at the corners.
