Hello to all. I am adding a 2nd story to my home. I have a 4" cast iron vent that y's off to pick up a galvanized 2" pipe for the bathroom below. I have a 2 part question. (A) The vent runs horizontally at a 90 for about 6 feet, then goes up through the roof. I would like to cut this out, relocate the run straight up from where the y fitting is now. Is this acceptable? (B) I would like to do this using 4" pvc. Would I be able to do this transiton with a sleeve? Thank you in advance to all for your replys.

Yes, it is acceptable. Actually, it is better when you eliminate horizontal section of the vent.

Yes, you can extend existing Cast Iron with PVC or ABS. Just use appropriate Fernco shielded coupling and keep the diameter of new pipe consistent with the old one.

Milo said it. You will be just fine.

it is better when you eliminate horizontal section of the vent.
Milo, Can you explain to me why it's better. I had always thought since it was simply air that bends and the position of the pipe didn't make any difference as long as the pipe was clear to the outside. Where did I go wrong? Thanks Milo

Joe,
You will transition from cast iron to plastic using a No-Hub Band or a shielded coupling, (see images) Good luck, Tom

Thank you gentleman for your timely feedback