I'll assume you are rendering RAM previews because you aren't able to get realtime playback and you want to test and tweak your animation.
Have you tried dropping down both the Quality and the Resolution levels (both found under the Render pop-up menu in Motion 4)?
Are you outputting through the Kona card to view on an external monitor? That will kill realtime playback and may affect preview renders as well
There are other things you can do to improve playback performance (like turning off thumbnails in the Layers tab and soloing layers) but it sounds like your main issue is ram previews.
--
Mark Spencer
Freelance Producer/Editor/Motion Graphics Artist
Apple-certified Master Trainer
Author, Motion 4 from Peachpit Press
http://www.applemotion.net
