Specify maximum number of sides in impoly

I'm trying to draw a polygon on a figure with a maximum of 4 sides per polygon, but I'm not sure how to limit the number of sides a polygon can be. Also, if the user draws 4 lines but the 4th doesn't connect to the beginning of the 1st line, is it possible to automatically connect the two lines? Does anyone know how these two things could be done?

This is what I've got so far:

vidObj = VideoReader('final1.mp4');
vidHeight = vidObj.Height;
vidWidth = vidObj.Width;
s = struct('cdata',zeros(vidHeight,vidWidth,3,'uint8'),...
    'colormap',[]);
k = 1;
while hasFrame(vidObj)
    s(k).cdata = readFrame(vidObj);
    k = k+1;
end

% select ROI
figure('Name','Please Click to Select ROI');
imshow(s(1).cdata)

l = 1;
numBoxes = 4;
h = zeros(4,2,numBoxes);
pos = zeros(4,2,l);
C = {'b','r','g','y'}; % Cell array of colros.
while l <= numBoxes

      api = iptgetapi(h(:,:,l));
      api.setColor(C(l));
      api.addNewPositionCallback(@(p) title(['Draw ROI for box ' l]));
      h(l) = impoly();
      pos(:,:,l) = ceil(getPosition(h(l)));
      l = l + 1;
  end