My laptop won’t display the full capacity of my RAM

How do I make my system to display the full capacity of RAM on board? My Laptop come with 4 gigabytes DDR RAM but when I checked the computer properties, it only shows 2.3 Gig RAM. This is really having a serious impact on my laptop efficiency. Playing some high graphics game has lately been nearly impossible because the system is now somehow slow. Could it be that the RAM is bad? why is it have impact on the functionality of the system?
