In Cohen-Sutherland clipping, it assigns outcodes to the points on lines that are drawn to specify their orientation relative to the clipping window.
The diagram on the Wikipedia entry for Cohen-Sutherland Clipping explains it pretty well:
left central right top 1001 1000 1010 central 0001 0000 0010 bottom 0101 0100 0110
So in your example,
p1 would be left, central, so its code would be 0001.
p2 would be central, bottom, so 0100
p3 would be central, central, so 0000
p4 would be central, top, so 1000