I played all the assassin's creed games on a worse rig, ac4 ran at 15 fps at lowest settings.
Intel dual core e2200 @2.20 ghz
ram 2 Gb dual core
same gpu as yours.
You will be able to that game better than me at least, my friend had 2.80 ghz and hd 5450, he play the game much better than me.