BMW uses brake pad wear sensors to monitor the condition of your brake pads.  Although your BMW has eight brake pads, only two of them are monitored for wear - the driver's side front inboard pad and the passenger's side rear inboard pad.  Thus, you should only order one front and one rear sensor.  When a monitored pad has worn down below 2mm of remaining friction material, the rotor chews through a small plastic bump on the sensor and slices a wire in half.  This causes an open circuit, triggering a warning on your dash, either an idiot light or a "CHECK BRAKE LININGS" text message.  Once a warning has appeared, the sensor has been destroyed and must be replaced.

Each Centric brand replacement sensor comes with a short wire harness that clips into your BMW's main wiring harness.  The other end (shown below in the enlarged gray box) clips into a slot at the top of your new brake pad, with the small plastic bump facing TOWARD the rotor.

E46 M3 sensor shown - your BMW model's sensor may have a different appearance

After replacing your pads and wear sensors, the dash warning will still be illuminated.  However, you do not need to go to your BMW dealer to have the warning message reset.  To reset the brake warning, turn the ignition key to the run position, but do not start the car.  After about one minute, the brake pad warning will reset.