Hi , I have a basement roughed in for a new bathroom . I have a 4 inch for the toilet , and a 2 inch which was supposed to be for the sink . Does this stub out for the sink need a separate vent line , or is it vented through the floor ? What would be the best way to plumb this sink , I am used to a horizontal pipe coming out of a wall . Thanks in advance for your help . Bob

To answer your first question, the sink CANNOT be vented under the floor so you do have to provide a vent for that pipe, and the toilet.

the pipe may be a wet vent for the toilet??
it must be extended thru the roof or tied into an existing vent stack.
if automatic air admitance valves are legal in you area, you can use one of them.

Our code requires use to drop down a vent for future fixtures. Have you looked for a capped pipe sticking down thru the joists ?
There needs to be a 2 X 4 wall framed around the pipe, a sanitary tee installed for the sink drain, and the vertical piping extended up to an existing vent or thru the roof.