I'm thinking 2023 Holiday. That would give it roughly a 4 year development cycle from when it was scrapped. I could see Switch 2 coming out that holiday season and Prime 4 being a cross gen launch title. I'm still holding out for my pet theory/fervent wish that Switch 2 will be PS4 Pro level of hardware and VR capable as a third "Switchable" form factor, and that Metroid Prime 4 will be the launch title that sells that experience by letting the player feel like they're in Samus' suit.
I like this. I like this a lot. Not really a fan of VR, but i'll make a big exception for Prime 4 (or any Prime game for that matter). I'm also betting it'll be a 2023 release, possibly alongside the Switch successor.
I'm not huge on VR either, but if it were just one form factor of three and not the focus, and Metroid was there to sell the VR factor, it could work. And for little 12 year old me, Prime was a mind blow with feeling like I was inside the suit, and I think a VR mode for Prime 4 could help me recapture that magic.