Meet the FlexLatch: a cat door holder that lets your cats in but keeps the dogs out of certain areas. It can hold most doors open 4-inches. You simply have to hook it onto your doorknob and the rest is easy. The FlexLatch Cat Door Holder gives easy access from either side of the door.
A 5.5-inch XL version is also available. You can order this in multiple colors. More information is available here.