By the end of The Fantastic Four's fourth year, the Marvel Universe is full born and on the verge of its maturity. The Fab 4 had become the nucleus for an expanding MU which included the Avengers, X-Men, the Howling Commandos, and individual series for Thor, Iron Man, Giant-Man, Hulk, Doctor Strange, Daredevil and of course Spider-Man. The Marvel Universe became more than the sum of its parts with crossovers becoming more and more commonplace as the shared world of these heroes became more detailed and rich with each subsequent story.
The FF start the year with a two-part epic which pits the Thing against the Hulk and brings the Fantastic Four in conflict with the entire Avengers team. Soon thereafter Doctor Strange is contacted to help solve a problem with the Sub-Mariner who has once again kidnapped Sue Storm. The very next issue has the Fab 4 mix it up with Professor X and his mysterious team of mutants, the X-men to do battle against the Mad Thinker and his Android.
Then the team end up in space again as the Red Ghost and his Super-Apes return and the all end up on the Moon. A new villain rises from the dead as the alchemist Diablo is discovered and unleashed by the team unwittingly. No sooner is his menace ended than Doctor Doom returns with some help from old FF villain Rama-Tut in the FF annual. We also get a rich origin story for Doom as he is elevated above nearly all the arch-enemies of the team to a status as their absolute number one opponent. And speaking of number ones, the original FF enemy the Mole Man shows up again to confront the team aided by his army of Moloids.
We learn more about Sue and Johnny Storm when the secret of their father is revealed and he returns in a most mysterious manner, as does an old enemy. Speaking of old enemies, the Sub-Mariner returns yet again and this time he's not the worst thing from beneath the sea as Attuma arrives on the scene. The FF are up against a man armed with a real-world super-power -- extreme wealth -- when they take on Gregory Gideon.
And then comes the weird and powerful Dragon Man who serves the returning alchemist from ancient days - Diablo.
And finally the year wraps as some of the Human Torch's old foes, the Wizard and Paste-Pot Pete (who re-dubs himself "Trapster") join forces with the Sandman to form the "Frightful Four". Their fourth member is Medusa, an enigmatic woman who we will learn eventually is an Inhuman. For now she is just another baddie.
And that's the fourth year of the Fantastic Four. The team will enter a new phase in its fifth year, a more leisurely and more complex storytelling will take hold and the concepts will become, if anything, bolder and more dynamic. But in its early days we see the FF go from a fascinating comic about more realistically drawn adventurers to become the hub of a larger scheme which hums and lives even in the modern day. No small achievement.