Not 100% sure, but I would imagine the CO Sensors would be on a different Loop than the Smoke Detectors.
Maybe the Drawing is showing the 4 Wires as:
* 1 Pair = Power,
* 1 Pair = Zone Loop.
Best to do here is an RFI (actually, an RFC) to whomever drew the plans you are referencing.
Maybe attempt to get an "Expedited RFI", by mailing / faxing / E-mailing / calling the designer directly, in conjunction with sending the RFI through the standard Red-Tape Channels.
Nevertheless, I would suggest requesting clarification from the designer, and verify the design intent conforms with all Local Codes that the AHJ will be referencing (Building, Electrical, Mechanical and Fire Department).
Looks like I may have opened the proverbial "Can 'O Worms"!!!