WARNING: The following contains spoilers for Thor #4 by Donny Cates, Nic Klein, Matt Wilson and VC's Joe Sabino, on sale now.
In Donny Cates and Nic Klein's Thor, the God of Thunder begrudgingly adopted a new role as the Herald of Galactus. Shortly after Thor was crowned king of Asgard, Galactus crashed onto his realm, warning of the coming of the Black Winter, the very end of the universe itself. By becoming the Herald of Thunder, Thor took on a mission to lead Galactus to five planets that will power the Devourer of Worlds up like never before so that he has a chance of defeating the Black Winter.
Unfortunately, Thor's actions as herald have landed him in the crosshairs of his old friends like Beta Ray Bill. While Bill's powers were no match for Thor's, the God of Thunder was near victory... until Sif showed up to put a stop to the fight.
Now, in Thor #4, Sif makes her point by sending Mjolnir away. And Thor's adoptive brother Loki ends up finding it and lifting it himself.
At the end of Thor #3, Lady Sif interrupted her duties as the all-seeing gatekeeper of Asgard to stop Thor's brutal beating of Beta Ray Bill. She warned him the only way to get to him would be through her, and that's exactly what the Herald of Thunder attempts to do at the start of Thor #4. With more pressing matters to attend to, Thor throws his hammer at Sif, but she activates the Bifrost and sends Mjolnir crashing down on another planet: Jotunheim, home of the Frost Giants and now, Loki.
At the end of the War of the Realms, Loki was crowned King of the Frost Giants. In Thor #4, we find the God of Mischief drinking on his throne. But he is soon interrupted by the arrival of Mjolnir, which smashes onto the ground next to him. "Interesting..." he initially remarks. Then, after debating the meaning of Mjolnir's arrival, after wondering if it's some kind of test or trap, he slowly reaches for the handle, and he does the seemingly impossible by lifting the hammer. "Very interesting," the God of Mischief says, as lighting cackles around the enchanted Uru. But before he can further wield it, Mjolnir is called away by Thor, and it flies back into space and returns to Thor's grasp.
Even though Loki has usually been one of Thor's greatest rivals, the God of Mischief has finally joined the very short list of characters who are worthy of wielding Thor's enchanted hammer.
Recently, in the pages of the short-lived Loki miniseries by Daniel Kibblesmith and Oscar Bazaldua, Tony Stark dared the God of Mischief to work towards becoming worthy of lifting his brother's hammer, but it was meant more as a taunt than an actual challenge. Still, the cover of Loki #1 featured the title character with Mjolnir in hand, teasing that perhaps the character was growing worthy.
What's more, this might connect to one detail revealed in Thor #1. That issue revealed the hammer was growing heavier for Thor, and it was Loki himself who noticed his brother's grunt when lifting Mjolnir. While the reason Mjolnir is growing heavier for Thor remains unknown at this time, it could be a direct correlation as to why Loki is now able to lift it. As it becomes heavier for Thor, it could become easier for Loki to wield it through some means.
After Loki briefly lifts the hammer, Thor #4's narration states that this "is a story for another time," which means that it may be explored further down the line. Since Thor is busy being the Herald of Galactus and King of Asgard, it might not be long before he gives up his God of Thunder title to Loki.