The universe of Earth-Four is the original home of the action characters from Charlton comics, after being brought by DC comics and integrated to the Pre-Crisis DC multiverse.
Earth-4 was created by Marv Wolfman, Robert Greenberger and George Pérez.
Note:Originally the Charlton characters were thought to be used by Alan Moore in a new project called Watchmen, but the DC editors vetoed the use of those characters, because they feared than the characters would be unusable after the story conclusion and Moore reworked his project with new created characters but based strongly in the Charlton archetypes.
Peter Cannon aka Thunderbolt, the character than inspired Ozymandias and who was an inhabitant of Earth-Four, didn't reappear in the new Earth-4, because their rights returned to his creator. The analog role of Ozymandias in Pax Americana was assumed by the character of President Harley.
Pre-Crisis / Earth-Four
The existence of this universe was brief. Earth-Four and the Charlton characters were introduced in the DC multiverse paradoxically at the end of their history in the crisis of Infinity earths. The heroes of Earth-Four along with the main earths could save their world time enough to make his world become part of the composite earth born from the crisis effect.
The Charlton characters from than point, along with their main metropolis, Hub city, became inhabitants from the DC Universe, with different degrees of success. However, until the next millennium, the Charlton universe would had a new chance to bright by their own force.
The New 52 / Earth-4
After Infinite Crisis and the following rebirth of the multiverse a new version of Earth-4 debuted in 52, an Earth-4 than resembled the Charlton characters universe. However in Multiversity, Grant Morrison played an inversion with the Watchmen-Charlton connection and Earth-4 became an analogy and homage to the noted graphic novel, with the original characters being played as their doppelgangers, the Watchmen characters and the context of the world including the existence of only one overpowered super-being.
Major Story Arcs
The New 52 / Earth-4:
Pax Americana: "In which we burn..."
The history in Pax Americana, the name of the team of superheroes, follows the life of the President Harley, and his rise to power, using the superhero idea as his platform. To reach the dreamed goal of a global peace, he plotted a crazy plan which include his own assassination and resurrection using the powers of Captain Atom.
The superheroes or action heroes in earth 4 started in the 70s with OSI super agents like Dan Garret and normal citizens like Yellowjacket as a mystery man. In later years new masked mystery men would appear, being sponsored by the charismatic Harley. Their number would grew and their power would rise with the incorporation of Captain Atom and the lethal super soldier Peacemaker. They would stop terrorists and super-villains in colorful clothes to distract the public from the country troubles.
The wildcard Question would separate from the team and would follow his own morals and instincts which would lead him to uncovered a conspiracy that affected the girlfriend of Peacemaker, Nora O'rourke, killed at the hands of Sarge Steel and the mystery of the algorithm 8.
The assassination of the president Harley was made by Peacemaker himself, the President bodyguard. However his resurrection didn't happened. Isn't clear however if the plan was ruined by the invisible hand of the Gentry intervention or if Harley just wanted to be punished by the crime of killing his own father, Yellowjacket, one of the first superheroes.