drywall in new master bath

I was browsing this forum and a question arose about my waste pipe. i have the old 4" cast iron waste pipe (w/ lead in each connection) going into my basement floor. My tub, sink and toilet drain in to it. I have the old cast clean out and can not get it open to clean. It still drains fine but i know it hasn't been cleaned for years. i am going to gut my bathroom soon and want to go pvc on all my drains. I would also like to get rid of all the cast iron pipe. This main cast iron pipe goes all the way up into my attic and through the roof. My question is can I cut out 3-4 sections of the main cast iron pipe in the basement so that I can replace it with 4" pvc. If I take these sections out will the remaining cast iron pipe above come crashing down without support until I get the new pvc pipe in? Do you need to support it? Thanks for your input!

I have two bathroom exhaust fans that are presently venting only to the attic. I have a few questions concerning properly venting them to the outside. Unless I go vertical (prefer not to to avoid roof penetration--but could be persuaded to --it is a ranch with a low sloping roof. A vertical run would be 4-5 ft.. Plus in a month or two will have snow on roof in central NY) the horizontal runs might be 10-15 ft. Both enter into attic approx 5-6 ft apart. Is 10-15 ft too long? Is there some way to connect these together in the attic and have just one vent to the outside? How so? Is it preferable to exit at gable end of house, rather than under eaves where the exhausted air (must be very tired by this time - ha ha) could re-enter attic thru the eave vents? Any reasons in favor or against using pvc pipe rather than smooth metal. Might be easier for me to work with and provide some sound dampening and insulation? Any responses and insight are appreciated. Thanks!