I was playing Halo 4 multiplayer with some friends last night and I noticed that certain players were harder to kill than others. After one has killstreaks do you become easier to kill or does the difficulty to kill players change?
The only thing that significantly changes how hard an opponent is to kill is the Overshield which can sometimes be earned through Ordinance Drops. However, Overshields are somewhat rare and the effect is obvious - the player glows bright green.
Imprecise weapons like the assault rifle will do less damage at range owing to the bullet spread. Obviously, the further the target is away, the more likely you are to miss with any weapon. Vehicles can offer some level of protection just due to their size and its effect on absorbing some shots. However, you can still kill a person with the same number of shots, if you hit the person riding in the vehicle rather than the vehicle itself.
Other than that, everyone has the same health/shields and goes down in the same number of shots from weapon fire.