First off, do you actually mean clematis paniculata? This is a New Zealand clematis that is typically grown in a greenhouse and which would not be hardy in your zone 4b. I am thinking you are actually referring to clematis terniflora or sweet autumn clematis. If so, realize that this clematis will get huge and a 15 inch diamete stump might work depending on how tall and excavated the stump will be. One thing to consider however is that the stump will eventually rot out and how are you going to deal with the plant and its roots when it does?