Venting basement bathroom??

Hi all..I have done an extensive search and just can't find an answer to what I am after. Hopefully I will be able to explain this clearly. Anyway, I am in the planning phase of finishing off my basement..just trying to get an overall layout. A question that I have is how to vent the half bath. As far as I can tell there is a "rough in" for a half bath. There is a capped 4 inch pvc pipe (toilet drain?) extending about 10 inches above the concrete floor, and a capped 2 inch pvc pipe (sink drain?) as well. Also, the main waste drain (3 inch?) from the above floors comes down about 4 ft from stubs and goes into the concrete floor. All that I can guess is that the roughed in drains connect with the main waste pipe somewhere beneath the concrete and then out to the septic. The concern that I have is that there is not a vent pipe that goes from the basement up to the roof?? Is this somehow vented using the main waste pipe (which is vented at a few points on the floors above)? There is another half bath just above the proposed basement bathroom which is vented (2 inch pipe) to the roof. I will post a few pics shortly.
Thanks in advance for any comment/suggestions!