There are 8 tiles in which the c4 elements alone are colocated, while the c2 elements are either in their original positions or swapped.
These tiles look very similar. They can be distinguished more readily if the weight function is plotted (as below). Here the purple areas shows those parts of the tile which cover the whole of the corresponding area of the plane, and the greenish areas those parts of the tile that cover 50% of the corresponding area of the plane, and the relative position of the filled and void regions is more readily seen.
© 2015 Stewart R. Hinsley