Second Story Bathroom Addition

I would like to put an additional bathroom on the second floor of my house. Presently, I have a single 4" DWV serving fixtures on the first floor. Am I correct in assuming that I would not be able to drain an additional watercloset, shower and lavatory into this stack? Must I run a parallel stack and connect at the base of the existing stack. As a remodels go, the possibilities are all pretty ugly. Any advice appreciated.

You are correct, a new drain line, (sanitary stack) needs to be brought up to the second floor to pick up the additional (new) fixtures. The stack that is there now is serving as a stack vent for the first floor fixtures.

Thank you..., a followup:

Thank you for you your kind reply. I have a follow-up question:

The basement of my house has a wye on the DWV 4" stack that is connected nearly at ground level and fitted with a fabricated trap/receptor standpipe that serves an automatic clothes washer. A washer is three DFU units by Oregon standards and shares the waste wet vent serving the bathroom above it. Is this OK and where should this be located in relationship to the second floor stack entry into the existing stack (above or below)?

Thanks again, Mark