First, we don't know anything about your project, your render settings, your operating system or your version of AE. This is important information that will help us point you to a solution.
Second, you should not be pre-rendering elements to MP4 files. MP4 is a lossy inter frame codec that takes a boatload of processor power to decode and is really only suitable for things like web delivery but not suitable at all in a production pipeline. You should use a lossless or nearly lossless production codec like Jpeg2000 or PNG compressed Quicktime or an image sequence.
...and you're using what version of AE? Right down to the last decimal place, please.
And you're running what operating system? Right down to the last decimal place, please.
And is your source video mp4, or are you trying to prerender to an mp4, which is a really lousy idea?
And are you trying to use multiprocessing, which can be kind of a crap shoot with 12 gigs?
And if you ARE using multiprocessing, how do you have it set up?
And what affects are you trying to use with the footage?
And are you stabilizing the shot or rotobushing it?
Thank you for the info really appreciated!
Im using Windows 8.1 and I have After effects CC