Defensive weapons have to be S4, to balance the new changes.
People were complaining that tanks were too fragile in 4th Ed, so GW made them tougher - new glancing/penetrating hit tables and cover saves. This is good, because you don't end up with tanks getting popped before they can even move/fire like you do now.
But to balance it they had to make them weaker to close combat (because otherwise tanks would be too powerful, and at least engaging a tank in CC requires some forethought and tactics to arrange), which is why CC units hit tanks on their rear armor automatically. And why defensive weapons are S4 - because if you want to fire all your guns you run the risk of CC squads hitting you automatically when they reach you. Thems tactics.
Without these changes, tanks would be unstoppable, moving 6" away from approaching CC squads every turn while firing every weapon AND being extremely difficult to kill from range.