Earth-4 is a reality in the DC Comics Multiverse. It has been described by writer Grant Morrison as a combination of the Watchmen universe and the Charlton Comics universe, with the characters that the Watchmen heroes were based off of standing in for their counterparts (Blue Beetle as Nite Owl, The Question as Rorschach, etc.).
This Earth is the modern counterpart of the original Earth-4, which contained the Charlton characters before the Crisis on Infinite Earths, which predated Watchmen. The new Earth-4 first appeared in 52 #52, where it was only depicted as a world containing the Charlton characters. However, in Grant Morrison's miniseries Superman Beyond, the Captain Atom of Earth-4 was no longer depicted as similar to the classic Atom of Charlton, but as a composite between Captain Atom and Dr. Manhattan named Allen Adam - aka Captain Adam. Morrison has since announced that he will be writing a series set on Earth-4, saying "I've just been doing an Earth Four book, which is the Charlton characters but I've decided to write it like Watchmen. [laughs] So it's written backwards and sideways and filled with all kinds of symbolism and because of that it's taking quite a long time to write."