Runner

The Runner is a new type of zombie introduced in. It is the fastest of the non-boss zombies (aside from the spawned enemies: Worms and Puke Worms), making it harder to avoid. The quote that they are "hideous but quite weak" is highly misleading, as they still have higher hit points than shamblers of same mutation.

Apex Runners are notorious among some players, since their high melee damage makes some buffs like Extra Strong and Poisoned Limbs skyrocket their potential damage output, yet even an Apex Runner without any buffs can be a serious problem. Because Runners are very fast (an Extra Fast Runner can run at 6.75 m/s) and powerful, they should be handled more carefully than most other zombies.

A good way of getting rid of Runners is to use high power weapons, automatic, explosives, whatever damage type fits your genre to get rid of them before they hit you. Another good tactic would be to max out Fast Movement to avoid them catching up with you, though Runners with the Extra Fast buff may still reach you. This can be avoided however with maxed Adrenaline, unless you're Medic or Heavy.

Trivia

 * Considering how much of SAS 4 is clearly influenced by the Alien franchise, the Runners may be a reference to the Runner alien from Alien 3.
 * Its Evolved form has spikes. The Apex form has more spikes on it.
 * Once spawned, they will scream although with all other zombies they don't always make noise once spawned.