The line from my septic tank ( in Virginia) is stubed out with 4" pvc. Can I reduce it to 3" when it comes through the slab or say the first fixture i.e. toilet? I would like to vent through the roof with 3'' not 4"

It will depend upon which code you fall under. In my area, (Manatee County Florida) you can pick up a toilet with 3" under the slab. One county south you must use 4" under the slab and 3" on the floors above. Callyour local Building Department and ask. Our code calls for one 3" vent, usually the kitchen and all the rest can be 2". But before you install, ASK! Good luck, Tom

Thanks for the info I called the inspector and it has to be 4"