attic addition draining

I'm building a master bed + bath in my attic and was wondering if I can use my existing cast iron vent that goes thru the roof as a drain. I was going to use the 4" vent to drain for toilet and the 2" coming up from kitchen sinks for the master shower and sink.

Nope. If it is being used to vent fixtures below, adding fixtures above will convert it from a vent to a drain. Biggest problem will be the siphoning of the lower fixtures when the upper ones are used. You will probably need to connect all of your new bathroom together in 3" and run that 3" all of the way down. Then add a new vent through the roof for the new plumbing or you can tie into the existing 4" vent for the venting.

NO! But how the downstairs is connected to the existing 4" vent will determine how difficult it will be to provide a drain for the upstairs bath.