Many people consider "Assassin's Creed 3" to be a success simply because of the amount of copies the game sold, but not everyone measures success by that gauge, and having said that, "Assassin's Creed 4" is going to blow "AC III" out of the water (no pun-intended).
One of the biggest reasons why "Assassin's Creed III" was a bit of a dud in people's minds was because of the amount of build-up and hype surrounding the game.
With "Assassin's Creed III," people were told this was the biggest this and the biggest that, and that there had been hundreds of developers working on the game. All this did was build up unreachable expectations in the minds of gamers.
If this game can stay away from being promoted to death before it is released, then "Assassin's Creed IV: Black Flag" can be considered better than "AC III" and a smash success now.