Question: Trying to do cleanup on my system that's running out of space, I found 4 gig in my Cellar directory for homebrew. I am not sure what job the cellar is doing. Is it just a cache that I can safely delete or are those files actually being used?

Answer: 
The cellar directory contains all the packages that homebrew installs.
However it also contains every version that you have installed.

You can run `brew cleanup` to remove old versions of packages to clean some space.
I guess I shouldn't be kvetching about my 1.4G Cellar. The cleanup brought it from 1.5 to 1.4.