Matte no shadow no C4D é um desafio para usuários do 3Dmax, definitivamente não vale apena correr atrás de outra solução, instala o plugin Shadow Catcher e seja feliz.
Fonte: sirBull Everybody told me that the way to render out from Cinema 4D was in multi-passes. That you split the image up in different "passes", and put them together again in a compositing software for better adjustments in post production.