Firstly. Uninstall all that memory hungry rubbish that you installed at the first place. ie all the wannabe freeware. That will greatly increase your available ram. Then disable a lot of others at startup. (Do you really need for an app to run in the background from startup that you only use once a month or even less.)
If you have a 32bit machine, 4gb is your limit. In a 64bit machine you ideally want 8gb minimum.
You can also invest in other things (Samsung Evo SSD drives to run your main OS). That could have a bigger impact.
And lastly your antivirus program. Some programs really hog your memory 24x7.
If you're into heaving gaming then sure you want the latest gfx card, and ideally 16gb of ram and an ssd, let alone a fast cpu. But if for normal a decent i5, with 8gb of ram, 64bit os, and an ssd to just run your os would be plenty. Add a second hdd or external for storage if needed.