It could be that the movement animations are better matched to the reticule adhesion / aim acceleration in Halo 4, and that all of the utility weapons are hitscan.
I’ve read through the Halo Council threads and watched the testing videos for Halo 4 and past Halos. From these tests, we know that the movement mechanics are almost the same (so, the pages tsassi linked) between all of the Halos. Also, though much has been said about the magnetism in Halo 4, bullet magnetism in Halo 3 was not tons different for the BR and AR (Halo 3 Bullet Magnetism) - though definitely a LOT less for the sniper (same vid). Halo 2 visually seems to have a bit less for some weapons, but tons for the PP charged shot (Halo 2 Bullet Magnetism).
So though the movement mechanics are the same between all the titles, if Halo 4 better matched the movement animations to the reticule adhesion and aim acceleration, then aiming in Halo 4 would feel significantly easier without greatly affecting the actual probability of getting a hit (or miss from good strafing techniques). So this would cause an apparent difference - but not a real one.
However, the BR in H4 is hitscan, whereas the BR in H3 was ballistic. Even if the bullet magnetism is similar (as the vids seem to demonstrate), removing the requirement to lead shots increases the actual probability of getting a hit. So this would cause a real difference in hit probability.
Relative aim assist values between Halo 4 and past Halos is unknown, though of the past titles, CE had the most (Legacy Halo Aim Assist) while Halo 2 / 3 were similar, with 3 having the least. Visually comparing, H4 seems to be in between H2 / H3 and CE.
So I would venture a guess that some of it is real (same magnetism + hitscan = greater probability of hit) and some of it is apparent (movement animations + aim acceleration = same probability of hit, but visually “easier” aiming). Reticule adhesion and friction may also add a bit of hit probability relative to H3 / H2, but is likely less than CE.
In short, I’m not sure how much has really changed, and how much just seems to have changed. But I do think that the real difference is not as great as it appears to be.
If going to H3-type levels is desired, the most expedient way would appear to be reducing bullet magnetism to counter the added accuracy from hitscan, but leaving the other aiming mechanics alone. I really don’t think we would want to go back to ballistic weapons, as behavior of ballistic weapons is strongly dependent on ping times, while bullet magnetism is the same for all players.