@Andreww 

While you can set the C4000 as an AP and set the wireless to match what your Gateway transmits its not recommended.   Because APs need to be connected dirrectly to the Router via Ethernet you may inadvertently create wireless interference by making the WiFi name the same  Also devices would be confused to which AP to connect to, the C4000 or the Gateway, causing device to radomly drop as they switch between AP.  Even if they are different because wireless operates on the same band there still might be interference  Think how apartment complexes have bad WiFi service becasue of Congestion in the immediate area.  Your best bet is to disable the Gateway's wireless and stick with just the C4000.