To play far cry 4 you will need to upgrade your graphics card. For Nvidia, any card below a x50 is not meant for gaming. (example: GT 620 = bad, GTX 650 = good, GT 730 = bad, GTX 780 = good) To play far cry 4 I would upgrade to at least a GTX 660 for a good experience.
Far cry 4 also requires at minimum 8GB of system RAM.