You might like to add the cabin is also protected by 2 positive pressure relief valves in the event of an overpressure. The 2 valves are located on the fuselage forward of the l/h wing. One will open if diff pressure reaches 9.25 psi and the second will open at 9.75 psi if the first valve fails. If any of these 2 valves ever open due to overpressure they will reset but the butterfly flaps on the fuselage will remain deployed and can only be reset by maintenance.
Correct technical term is Cabin Pressure Controller on this aircraft not Cabin Altitude Controller.
NAI control switches on the overhead panel P5 are optional. Some operators choose square pushbutton switches and others use toggle switches with an AUTO position. In AUTO the NAI valves are controlled by the ice detectors. During Rev thrust the NAI valve will close.
As mentioned earlier the WAI uses bleed air for leading edges, only operates when the leading edge flaps are retracted. The WAI can also have an AUTO position which again uses the ice detectors like the NAI for control.
All -400 aircraft i have worked on have 2 ice detector probes.