I think the problem with Assassinations in Reach, and I could be very very wrong about this but it fits with my experience at least, is that the game might be having the host determine whether you've held the button down long enough. So transmission problems between you and the host could make it seem as if you held down the melee button longer than you actually did, causing an Assassination. If this is the problem (and this is a huge if), then it's possible Halo 4 has corrected it.
Yeah, even if you pressed the button or bumper for 0.01 seconds it could cause a assassination, because of the delay between players. Mostly it ends up with those very very fast and buggy assassinations, which are faster, then a backsmack and you still get the medal.
My problem (and many other may have this problem, too) with Halo 4 is, that we didn't saw, how Halo 4 performs over Xbox Live. All those LAN games looked very good, but we know, how bad Xbox Live can be sometimes.
I'm making the assumption, that those network tests, weren't tested under extreme conditions. You can correct me, if I'm wrong, but I don't believe, that they let people participate, who have a wooden router, in those network tests.