Tying 4"x4"x2" all-hub into existing stack

I'm renovating my master bath and have a question on tying in a 4"x4"x2" all hub waste connection into the existing stack. My intent is to leave the remaining existing rough plumbing as-is but if I have to remove, then the new will trace the same route as the existing. Since I'm converting the old tub area into a single shower, I have to change out the 1 1/2" p-trap into a 2". Where I'm confused is the tie-in of the new PVC 4"x4"x2" into the existing cast iron vent stack. I know I have to use the reinforced hub couplings since the connection is above ground. Given the diameter of the 4" hub is wider than the 4" cast iron, do I add a length of 4" pvc pipe into each of the 4" hub sections to create a better match of the pipes to be connected with the reinforced hub. I've attached a picture to give you my idea. To me, adding the additional 4" wide section make the transition more clean. Thoughts?

http://i114.photobucket.com/albums/n245/esuggs/ShowerStack.jpg


Santee

They make a PVC to CI no hub adapters you will need a short piece of PVC
pipe between tee & CI.Use no hub couplings.

Better make sure you support that stack on both ends!

Good point, I'll have the walls open. We're in a single story house with good attic access. Perhaps I should just replace everything down to the waste line, minimizing the modifications.

Good idea to replace it all.