could be a number of things
whats your system .cfg ?
dual core CPU's are not supported by Far Cry 4 nor are 32 bit operating systems
do you have an AMD based GPU , if so be aware there is a problem with Far Cry 4 and the current beta Driver version 15.11 to fix it you'll have to roll your driver back to beta version 15.10 Drivers or the WHQL driver 15.7
please see this thread
http://forums.ubi.com/showthread.php...9#post11152379
and check out what it says about back round applications .. as well as how to contact Ubisoft Support