It was a whole different turn for the canon series of Marvel's What If..? as this run not only took us to the darker side of Doctor Strange, but it also showed that these alternate universes coexist with each other forming some kind of connection with those in the story. It weaves a whole reality and this time, the ending of Episode 4 explained what happens with the branch that was made.
Warning! This portion may contain spoilers for Marvel's What If..?. Read at your own risk!
The Ending of Marvel's What If..? Episode 4 Explained
In Marvel's What If..? Episode 4, What If... Doctor Strange Lost His Heart Instead of His Hands?, we see Doctor Strange face the most formidable enemy he would ever be going up against in his life – himself, and the while it is uncanny to see, it is the very first time that a character connected to Uatu, the Watcher, and it fits that it is the Sorcerer Supreme.
The episode has shown us that the reason Strange looked for a new purpose in life is not that he lost his sense of touch, he lost the love of his life, Dr. Christine Palmer, and despite knowing how time works and how it goes around, he still cannot cope with the loss of his heart that he studied for years looking for answers on how to get her back to life.
As explained by The Ancient One, in this timeline, Christine's death was the start of the journey of Strange, and without such an absolute point in time, he would not have defeated evils like Dormammu and the dark dimension. Still, grief-stricken, one part of Strange did not hesitate to look into the possibility of bringing Christine into life. Hence, the Ancient One had to make a tough call – tap into the dark dimension to let Strange's two versions coexist.
However, as we learned, the paradox is not that simple. What the other one does will affect the life of the other. This is why when the dark version of Strange consumed so much power from various magical beings, the other who was just guarding the Sanctum had his reality affected as the dark dimension is finally spilling up into the other.
To stop the other, Strange must face himself. He had to battle his desire with the delusion that he could bring Christine back to life without sacrificing a whole universe for it. He became selfish while the other became selfless. O'Bengh, after all, was right. He was only half the man he was supposed to be. In the end, while he was able to bring Christine back to life, he got so consumed by the powers of the mystical beings and looked like a monster.
The universe he was existing in was obliterated and for the first time, interaction with Uatu was made. Uatu claims that he is no god, but so isn't Strange, so the universe that carries their reality just collapsed since he can only watch, not intervene. It shows the limitations on what Uatu could do, on what he could only control.
In the end, Strange was imprisoned in his own failed universe and he had to accept the fact that it was his own doing that brought said reality to the brink of collapse.
Marvel's What If..? Episode 4 is now streaming on Disney+.