Connected the Charlton Characters and Costumed Crusaders

Unlike on other parallel planets, the characters of Earth-Four... also known as the Charlton characters... operated mostly on their own detached from one another. When first integrated into the DC Multiverse, a handful of them appeared to battled heroes from Earths One and Two while themselves under the mind control of Psycho-Pirate during the Crisis on Infinite Earths.

Among these were the quartet known as the Sentinels of Justice, consisting of Captain Atom, Blue Beetle, Nightshade and the Question. Alongside them, Peacemaker also appeared during this battle, as well as Peter Cannon aka Thunderbolt. A fifth costumed crusader from the distant past, in Judomaster, also materialized into the present due to affects of the Crisis.

This was the first occasion where the latter three were shown as contemporaries of the main "Action Heroes", as they had been called. Missing was Son of Vulcan, who appeared after the Crisis with them.

Some years prior to this, there were connections chronicled with other notable individuals of this world. For example, Nightshade as a little girl was trained in karate by an adult Tiger, who had himself been the child sidekick of World War II hero Judomaster! An so, the heroine was a third generation superhero due to the tutelage of her master and his master before that!

And during those early days of that tragic war decades prior, Judomaster faced his arch-enemy in the Nazi agent known as the Smiling Skull. After his government collapsed and his cause had ceased to be, the Skull later engaged in criminal activities which brought him to the attention of government agent Sarge Steel, whom he would repeatedly face. 

And some time after he had dealt with the Skull, Sarge Steel became involved in a case involving the original Sentinels of Justice, with its members Helio, Brute and Mentalia. While it has not been officially confirmed, it is believed that this first incarnation of the Sentinels during the 1960s inspired later heroes in Captain Atom's team which we've previously encountered. And of course, that team's Blue Beetle was tied to his two predecessor Beetles.

Other heroes also existed on this world, although crossovers were not a regular occurrence. On one occasion, Blue Beetle and Peacemaker teamed up with three established heroes known as Superguy, Captain USA and Green Spider of whom not much is known.

Like the Fawcett Earth-S universe that existed in a parallel plane of existence to it with its Squadron of Justice, Crime Champions Club, and Marvel Family, it was a decidedly more divergent continuity from the more main stream Earths.