Can I reduce a 4" toilet/sink drain line to 3" at the main stack? Toilet is approx 25' from stack.

I'd feel a lot better if you were increasing from 3 to 4" instead of choking down from 4 to3". I wouldn't do it but let's wait until the reast of the experts chime in and see what they think.
What say you guys? Tom

I agree with Tom... I wouldn't do it. Simply put, if someone tried to snake the drain line with a large cutter head and ran into the reduction it could definitely lead to a binding of the cable (if stuff stuck on the cutter head of the cable as it gets pulled back... picture roots from a tree) or a breaking of the sewer snake head.

Good luck!

MARK