Spitter

Spitters are a zombie introduced in. They are weaker, but a little bit faster than the Shambler, and have four forms: Normal, Fetid, Evolved, and Apex. Spitters are ranged, and their "spit" leaves an acidic pool around for four seconds. Spitters also have a small amount of chemical resistance. Despite the lack of real damage when hitting the player, these zombies can deal a lot of damage over time with their spit. Spitters are most annoying when you are camping or in narrow points. Spitter acid becomes more and more deadly with each form.

Spitters with Extra Strong and/or Beserk modifier can be extremely deadly due to the increased damage from the acid pool and/or the increased amount of acid pools.

Trivia

 * The stats seem to double as the Spitters evolve, but Apex seems to jump suddenly in some spots and go over double the stat before it.
 * Although Fetid Spitters first appear at level 5 and stop appearing around level 45 (the first appearance of Apex Spitters), there have been many reports of Fetid Spitters reappearing at level 72. Many YouTube videos have been made about this. It is likely a glitch as Fetid Spitters should go extinct when Apex Spitters begin appearing at level 45. (Note that the first apperance levels are for singleplayer mode.)
 * Considering how much of SAS 4 is clearly influenced by the Alien franchise, the Spitters may be a reference to the Spitter alien from the video game Aliens: Colonial Marines.
 * According to Ronson, spitter blood can burn away Myraphilus bacteria.
 * The Spitter appears to be wearing a patient gown in the basic and fetid variations, indicating that it was hospitalized when it was infected. This may also apply for the internal organs turning into acid.