If you were to look at the sales, Halo 4 was easily a huge success. If you were to look at the amount of people who came back because of updates that 343 made, it was still a success.
If you were again to look at the amount of people who regularly play the game, still a success just not as a big as previous titles.
However, if you were to take into account the amount of flaws the game has, it's a mediocre game. But far from a fail.
Halo 4 kills other games like Halo Wars, Halo 2 and Halo 3 ODST in my opinion for a few reasons. Halo Wars was the first Halo game made by Ensemble Studios, and is their only game. Halo 4 crushed it in sales and people still playing. Halo 2 was the buggiest Halo game known to date, its multiplayer was completely broken, so many say that was good for the button combos, but that's cheating and not how Halo is meant to be, it's core is a balanced game, not designed for cheaters who exploit the game to win. Halo 3 ODST lacked any sort of proper Online Play, therefore Halo 4 automatically wins that, even though it didn't have the successful gametype of FireFight. Halo 4 for a first fresh game made by such a new team is fantastic, so many ideas were put together, even if they didn't work it shows that 343 industries honestly have so much on their mind and want to make their franchise as big and exciting as possible innovating with so many new things. This is something that all fans should be excited over, many complain about the lack of change of games, and 343i are here to make their games something new and special. Even though not everything worked, they tried and learned from their mistakes and promised to make their next Halo title much better, and that's all you can ask from a Dev team who doesn't have to listen to their community, yet do so anyways.
So in conclusion, yes, Halo 4 was a success.