Thanks to Thor 4 happening, Thor is now Marvel Studios' longest-running franchise on-screen. Sure, Robert Downey Jr. kicked off the overarching Marvel Cinematic Universe with Iron Man in 2008, but his solo series only lasted for a few years; Iron Man 3 released in 2013. However, it's worth noting that Downey Jr. himself continued to star in MCU movies up until Iron Man's death in Anthony and Joe Russo's Avengers: Endgame. And he wasn't the only one to say goodbye in that momentous film.
Other characters who got their own solo movies in the MCU's Phase 1 were Hulk (with The Incredible Hulk), Captain America (with Captain America: The First Avenger), and, of course, Thor (with Thor). Unfortunately for Hulk, Marvel Studios has not only opted to discontinue making solo movies for the character but also recast the role with Mark Ruffalo for The Avengers. Captain America fared a bit better. Marvel fleshed out Captain America's trilogy with movies in 2014 and 2016, but thanks to Endgame, he's also off the table for future installments, at least with Steve Rogers.
On the other hand, Thor is continuing beyond the Infinity Saga and into the MCU's Phase 4. With Thor: Ragnarok as an enormous success and Thor's popularity being at an all-time high, due in large part to his appearances in Avengers: Infinity War and Avengers: Endgame, it makes sense that Marvel could commission Thor 4 with Taika Waititi as director. But now that means Thor is Marvel's longest-running franchise. Kenneth Branagh's Thor released in 2011, so Thor 4's release in the 2020s means the series has gone on for at least a decade.
In addition to that, Thor 4 marks the first time that a Marvel character has received a fourth installment in their solo series. Even though they've all appeared in the Avengers team-up movies at some point, getting a fourth movie to call their own is a big deal, especially since the vast majority of characters from the early days of the MCU are now gone or going away. Tony Stark and Steve Rogers are done; Black Widow has a prequel movie coming up; Hawkeye is reportedly getting a Disney+ series to wrap up his story, and Hulk is still confined to other characters' films.
Thor, on the other hand, is the only character from Phase 1 that still has an open future now that he's no longer the king of Asgard and doesn't have any immediate enemies or concerns. That'll make Thor 4's story quite interesting, as it could potentially become the first of more Thor movies or the character's final installment before a new hero takes his place - perhaps Valkyrie or Beta Ray Bill.