As another user pointed out, why should be the chromatic polynomial of rectangle with deleted edge be: q^4 - 3q^3 + 2q^2 and not q * (q - 1)^3. A counter example: when q=2, we have two ways to color the rectangle with a deleted edge. Am I missing something?

I think fixating q as the number of possible ways to color the end points of the deleted edge leads to the wrong result.