Metroid is a franchise known for having low sales, it's not Mario or Zelda to be that hyped...
I'll preface by saying that I don't think The Legend of Zelda is a low-selling franchise, but when you compare it to Mario historically, it's like comparing Zelda to Metroid in a way. Both Breath of the Wild, and Tears of the Kingdom broke those conventions because prior to BOTW, no Zelda title ever sold more than ~9 million copies. Still very good, but a far cry to what Mario has done over the years. BOTW was the first real blockbuster Zelda title in that regard.
Some comparisons to be made with this too. On N64, Mario 64 sold nearly 12 million copies, whereas OoT sold under 8 million. On GCN, Mario Sunshine sold just under 6 million copies, and Wind Waker by contrast sold over 4.4 million. For Wii, things were about the same. Mario Galaxy 1, and 2 sold 12.8, and 7.5 million, respectively, while Twilight Princess sold 7.5 million, and Skyward Sword did under 3.7 million. Mind you, New Super Mario Bros Wii sold more than both Zelda, and Mario Galaxy games on Wii combined, a staggering 30 million copies.
And Wii U? Well...we'll just leave that one aside for now.
And then we get to Switch. FOR THE FIRST TIME EVER, a full-fledged Zelda title sold MORE than a main Mario game. Almost 32 million for BOTW, and a mere 27 million for Mario Odyssey. That just does not compute, and I can guarantee in early 2017, prior to the launch of Switch, and BOTW, if you had told people BOTW would sell more copies than the latest upcoming 3D Mario, you'd be laughed at. That's not to say BOTW wasn't hyped beyond recognition because it was. It was famously delayed 2 years because Nintendo decided they needed to put this game on Switch instead of keeping it exclusively on Wii U. It was a huge decision, but it paid off immensely.
Now, what does this have to do with Metroid? Well, we ultimately cannot say for sure how Prime 4: Beyond will sell. We can certainly look at what previous titles have sold (similar to what Zelda had sold over the years on different platforms), and make estimations, and other projections, but I think people did the same for BOTW prior to its launch.
I don't anticipate Metroid Prime 4: Beyond will have a similar scale of success like BOTW did for Zelda, but when we think about the hype surrounding Prime 4 from its infamous E3 trailer in 2017, to the equally infamous 2019 Dev update on restarting the game from scratch, to now the first actual look at the game just a week ago, with a 2025 release date, anything can happen at this point. Metroid Dread is the best selling Metroid title of all time (which isn't saying much unfortunately, given it is only 3 million), and I'd be surprised if Prime 4 doesn't exceed that. I'm not picturing BOTW/TOTK levels of success, but even 5-10 million would be massive for the whole franchise.
But yes, Metroid is historically known for low sales, in the same way Zelda historically sold less than Mario. We'll find out for sure sometime 2025.