Playing on the PC with a Nvidia Geforce 970 card, the game runs smoothly at max settings and to be honest, that alone is almost worth it. I've never seen such a beautifully rendered game in my 40+ years. It is just fantastic to run around in and look at, from the Tibetan-inspired Himalayan peaks, no other current game can touch it really.
Far Cry may be turning into a COD/GTA-like franchise due to its proven formula and emphasis on expensive voice-acting and movie-quality characters. Not really such a bad thing, but I'm not sure where they go from here unless Far Cry 5 is in an urban setting or something.
Far Cry 4 expands on FC3 with more wildlife, tougher and more enjoyable missions, side quests, and the addition of "Fortresses" to Outposts, in addition to its amazing visual upgrades (And FC3 is a stunning game as well at its max-settings). The Gyrocopter, hovercraft vehicle and expanded multiplayer modes add to the entertainment value.
As of late December/early Jan 2015, I don't have any problems running the game - it's smooth and bug-free except for the occasional visual glitch (i.e fish swimming above water after being shot at in a cave). Keep in mind that was a one-time thing for me in 20+ hours of gameplay thus far.
Could they have done more with the game to differentiate it from the familiar formula of its predecessor? Probably, I suppose. Does that take away from all of the above? Not really.
If you loved Far Cry 3, you'll love Far Cry 4. It's really that simple.