How to choose a processor core for rendering?

AE rarely will ever use all four cores, so this is pretty much a moot point. Many functions in AE are barely multithreaded nor does ist support any fancy parallel processing save for a few optimized effects. I suggest you research this forum and the web in general on this matter. There's literally thousands and thousands of discussions on "performance optimization" in AE and similar out there that explain all the details. With that in mind, AE's main process locking on to a single core is perfectly normal. It's just odd that in your case it's the last one instead of the primary one, but without more specific info on your system nobody can figure out why it behaves this way.

Mylenium