You can perform assassinations in Halo 4 if you really like to humiliate your enemy. You have to be careful though, because it is a longer animation, so it leaves you open to attacks when you perform an assassination.
To assassinate someone, you just need to be behind someone (or to the side but slightly behind them) and press and hold the B button.
The assassinations in Halo 4 are also a lot more elaborate than they were in Halo Reach, with special animations for assassinations with a Sword, Flag, even an Oddball. There are also special aerial assassinations as well, if you time it perfectly.