Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Inner Radius
PRIM_DISC_IRAD
Parameter: Outer Radius
PRIM_DISC_ORAD
Parameter: Disc Segments
PRIM_DISC_CSUB
Parameter: Rotation Segments
PRIM_DISC_SEG