I am planning on deploying PMP450 with a rack mount CMM4. I know I will have to use the 29VDC power supply to provide power to the AP but do I also need the 56VDC? I will not be deploying any access points or PTP that will require a 56 VDC.
No, you do not need the 56 VDC power supply unless you are connecting a radio that requires this voltage.
The CMM4 itself will draw power from either supply.