I’m nearly convinced that aim assist changes from game to game. Not just me personally, but others whom I’ve mentioned this to, have ideal feelings on the matter.
The next time you are playing a game, and your team is doing significantly bad, take a look at your aim assist. Either the game strips aim assist from you, or it has been dumbed down noticeably. This applies to the entire team in the game, but not the other. You can even view this in theatre. Why is this?
Vice Versa, the next time your team is dominating, check your aim assist, and you’ll notice that it’s performing exceptionally well. Why is this?
Opinions / Comments? I want to hear if others have experienced this, or have not.
Obviously, if your team is doing well, you are probably doing well too, which means your aim is better. Vice versa, if your team isn’t doing well, chances are you are playing as sub-optimally as them, and hence your aim is worse.
It’s all in your head. There is no reason any game designer would implement a gameplay element that irrational. Ultimately, after making such a claim, the burden of proof would be on you. Regardless of what you say, unless there is some hard data backing up your claims, that whole idea seems far fetched and naive.
So, in short: It’s highly unlikely that there would be such a mechanic in Halo 4. A much more plausible explanation is a human tendency to try to find an external excuse for all your mistakes. After all, it’s never our fault, is it? 
I highly doubt there is a system in place to make you do poorly. It isn’t a conspiracy or some way to blame 343i again for something else. Please don’t enforce this kind of thinking because it starts rumours and pretty soon people are going to be bashing 343i for something ridiculous they did not do.