correct me if I'm wrong, each project uses the amount of memory needed to render it. depending on the type of scene in question (radiosity, particles, hair, Newton .....).
If you need increase speed to render an animation process you can set a processor affinity in the task manager processes ( left click on MASTER64 process and choose set affinity ) In your case you can run 4 instances of A:M and stablish one processor for each instance.