Nintendo has cancelled the current development of Metroid Prime 4 after subpar results from the current team. Retro Studios has been assigned to restart the project from square 1. Naturally this will set Prime 4's release quite significantly, making me wonder if perhaps it will just end up being moved to Switch 2 in the end.
^ That's what my hope would be, whatever project they are working on (including that Star Fox Grand Prix rumor) would be announced/released this year while it wraps up giving them the resources to work in Prime 4.
After looking through the credits list for DKCTF (Switch) it appears Retro did that version all by themselves with no assistance from Monster Games unlike the Wii U version and credited 69 employees compared to 105 for the Wii U version and that includes a few extra individuals for "contract sound". So there at least another 30 Retro staff who are either gone or are still working at Retro on SOMETHING.