A new batch of rumored The Avengers 4 spoilers have hit the net that not only mentions Kang the Conqueror, but also puts a whole new spin on the title I was told for the movie, "End Game."
Worth a mention before we start is that The Avengers 4 spoilers come via 4Chan, but the site has been pretty close regarding their Infinity War rumors, the Star Wars: The Last Jedi rumors, and their Guardians of the Galaxy 2 rumors did prove legit.
Obvious huge spoiler warning for what follows.
Also note some of my thoughts are in the parenthesis.
The Avengers 4 Spoilers
It's said The Avengers 4 starts off with Ant-Man trapped in the Quantum Realm where he uses a Time Vortex to escape, and he travels to events before the first Iron Man movie and finds Hank Pym. The two create a "sling ring," which is a mystical object which enables the wearer to open a dimensional gateway to another location, as seen in the Doctor Strange movie and Infinity War. While Strange used it to to create a portal between two points on the planet (i.e. when he retrieved Tony Stark), apparently Scott Lang and Hank Pym's sling ring is able to time travel (assuming with the power of the Time Vortex from the Quantum Realm).
Cue title sequence
Iron Man and Nebula are shown on Titan (following the events from Infinity War), where the pair then make their way to Earth to re-team with the surviving Avengers.
Ant-Man then comes out of the Time Vortex, where the Avengers tell him about the "Snappening." Ant-Man informs them about his sling ring that can time travel.
Since Asgard is destroyed and Thanos wiped out the Asgardians in Infinity War, Thor goes off to Hel to build an army of fallen Vikings, with Hela also there (intrestingly enough, it was decided to cut a similar scene in Infinity War for Stormbreaker's origin).
Thanos then finds out about the sling ring that can time travel and sends his own army, which meets Thor's army in battle.
The battle sees Thor and Captain America getting killed (all said to take place in the first hour). It's said Thor dies in the first battle, but Captain America dies while fighting Thanos' army after time travelling, as Thanos also sends his army after the Avengers in the sling ring time travel portal/dimension.
The Avengers decide to use the sling ring to time travel back in time to collect the six Infinity Stones and stop Thanos.
Since Captain America died, they decide to travel back in time to recruit Cap from the time of Captain America: The First Avenger and grab Cap right after the Red Skull is transported by the Tesseract.
It's further revealed that The Avengers 4 features the Marvel Cosmic abstract character (at least in the comics), Eternity, who helps out the Avengers by giving them special gauntlets that lets them control their movements in time (note: The Russo brothers seemingly teased Eternity, and we can guess it's possible that Pym / the Avengers run into Eternity in the Quantum Realm).
The Avengers time travel to previous MCU movies in order to get the Infinity Stones: The Avengers, The Avengers: Age of Ultron, right before Thor, Thor: The Dark World, and Guardians of the Galaxy.
Hawkeye also travels to the events prior to the first Thor movie to get Odin's Infinity Gauntlet.
Regarding travelling back to the first The Avengers movie (as revealed by set pics and footage), the rumor has it that "the Battle of New York gets even worse."
Doctor Strange is said to escape the Soul Stone and use a sling ring to time travel.
Thanos learns about the Avengers going back in time attempting to retrieve another Infinity Gauntlet and sends more of his minions and army in what is a huge battle. The battle features Avengers from both past and present (or future) where half of the Avengers from the past and half of the Avengers from the future die.
It's said that the Hulk eventually gets the Infinity Gauntlet with all the Infinity Stones, but when he uses it, his arm explodes, and later the Hulk (or Banner?) gets a mechanical arm (it's also speculated that Banner and the Hulk merge for Avengers 4 becoming the smart Professor Hulk).
The post-credit scene for The Avengers: Infinity War showed Nick Fury paging Captain Marvel; The Avengers 4 rumor has it that she doesn't do much until the last hour of the movie.
Eventually, Thanos is defeated and is said to die, with the Infinity Gauntlet and the Infinity Stones destroyed.
The Avengers 4 isn't done there as Tony Stark absorbed the power of the Infinity Stones.
It's revaled that Eternity is actually the future version of Tony Stark, and that Eternity has been manipulating the timestream (for his own selfish ends), as Eternity changes events in the past to make most of the events happen.
It's shown that only Dr. Strange, Hulk, Red skull, and Thanos are the people with "free will."
In the future, Tony Stark became Eternity after he ascended his mortal body. The present Tony Stark is revealed to become a conqueror and to rule over the universe as a god in the future as well. The people remember him as Kang the Conqueror before he took over (sort of sounds simlar to Iron Lad from the comics).
It's revealed Eternity needed Doctor Strange to get rid of Dormammu.
The final 20 minutes or so are said to feature a fight scene featuring the remaining Avengers and Kang (Kang controlled everyone).
Doctor Strange then time travels right before Tony Stark becomes Kang the Conqueror and kills him.
The Avengers 4 ends with the remaining heroes happy the war is over.
The Avengers 4 post-credit scene is also said to feature Ultron waking up.
(note: h/t to a redditor who also did a write-up on The Avengers 4 spoilers, but my understanding of the rumors were slightly different.)