I have typically installed a 4/0 bare cu wire in pvc power ductbanks, but should the same apply to instrumentation (4-20mA and D.C. circuits) and control (120V A.C.) ductbanks?
4-20 mA does not require an EGC in the raceway. the size for a 120 control circuit is based on 250.122.
However, there may be job specs that require a 4/0. I don't see any reason for a 4/0 in either application.