MentalRay and Pixar's Renderman are both available for Maya 4, but there are no betas or freebies like the MentalRay for 4.5 unfortunately. Probably the two best renderers out there today...but the prices reflect that!
Look here for more info-
Red bellows of flame have blackened my stones
Convulsing my frame and cracking my bones
Hell's dragons of steel who roar in their chains
Crawl into my caves to suck out my veins.....
-The Mountain P.F.M.