the Q texture coordinate, quad stretched in 2 dims


I have a problem with OpenGL and the Q-Texture-Coordinate. My problem is that my Quad is not only streched in one direction, but in two. Here are some example coords(counterclockwise, beginning on the lower left):
(-10, -10, 0)
( 10, -5, 0)
( 6, 8, 0)
( -9, 10, 0)

I read already the (very good) article about the Q-Coordinate:
which covers only the the one dimension stretch. There is an example picture on the page.
It would be very kind, if someone could help me.