Here's why I suggest turning off multiprocessing: you don't really have enough RAM to take advantage of it.
4GB may seem like a lot, but consider that AE can use 2GB+ per processor with multiprocessing enabled. That's at LEAST 16 gigs on your 8-core machine... don't forget that the OS needs RAM, and so do any applications you like to leave open.
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA