vista is a pig and loves to get slow. but still 4gb should be enough.
The only reason you would want to expand your available ram would be if you are running out of it during regular use and that would either be a. your using more programs than available memory or b. your running background programs that are eating up your resources making less available system ram for your applications that you have in use.
Other things cause system degradation such as file fragmentation and excess temporary / junk files left on the hard drive.
1st, empty your trash can and try running the system tools that come with the PC to delete unneeded temporary files. Then run a hard drive defragging tool and let it defrag the system. Next get a good adware removal program like lavasoft adaware or spybot search and destroy, run this and remove all the threats it finds on a weekly basis.
uninstall unwanted programs, toolbars in your browser and little desktop apps that might be running in the background.
If you can get through these steps you will probably have a decent pc again.
Worst case, backup your files and reinstall a fresh copy of Vista or Windows 7 and your pc will run like new again.