Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Inner Radius
PRIM_DISC_IRADParameter: Outer Radius
PRIM_DISC_ORADParameter: Disc Segments
PRIM_DISC_CSUBParameter: Rotation Segments
PRIM_DISC_SEG