I'm running Virtualbox to run my server on CentOS
The thing is I have memory available on my Xserver 16 GB
And the max I can up is only up to 4GB
I don't know why I can't give it more?
I'm not a mac user, but are you booting up the host with the 64bit kernel? Is the program using the 64bit version?
Generally, a 4GB limit is an indication of a 32bit/64bit issue.
I found this article on VirtualBox's site with a user that is having the a similar issue. It may help you resolve your problem.