I'm just speculating here, but I think that when GT4 came out, PD did not consider the Cadillac Cien a production car, but instead considered it a "concept car", and hence is not allowed in this series. See Famine's reply dated 1/17/09 in the other thread started as "Supercar Festival?".
This seems to apply to the Audi LeMans quattro '03 as well.