After four comes five, but we'll get to know one or two other characters first, which of course will serve as components for the character for five and many others. 丨 is just a vertical stroke, but I like to call it the dinosaur bone. It has no inherent meaning, but serves as component for many other characters. Whenever you see a dinosaur bone in a character's painting, you'll now that a vertical stroke is meant.
As for the pronunciation: First of all, you don't need to pronounce this character. It has no meaning, so you'll never use it in speech (except if you want to refer to the character itself of course). If you want to write it using a Pinyin input method, you'll use its pronunciation "gun3", which translates to "gu" + "(e)n3" in the Gorodish system. "Gu" is for Gitta Giraffe, because, you know, initials that contain an "u" will be represented by animals—at least in my mnemonics. "(e)n3", a final, is represented by the ForbiddEN City.