I wouldn't say 96% of the time is "often". I would say "cops get off the hook virtually always".

Of those 4% who do get investigated, about 3 out of 4 don't get indicted, because the prosecutor delivers the case in such a way that the Grand Jury forgives them, even though in 99% of the non-cop related cases, the prosecutors manage to score an indictment.

So basically only 1% of cops ever get punished for their crimes.