The first thing you will probably need for using 4GB or more of RAM is a shift to 64-bit OS.
Beyond that,
But, going towards more RAM and moving to 64-bit OS has its own constraints.
A lot of the usual stuff is not yet quite stable (or even available in some cases) for 64-bit systems.
Worthwhile question -- Do You Really Need More Than 6 GB Of RAM?, even 4GB
