Metroid Prime 4 is a mega-mystery at the moment.
You know when studios get up on stage and announce a game at a tradeshow -- they usually clue us into what developer is handling it, or, at the very least, provide that information in a press release after the fact. Not so with Metroid Prime 4, as the development team is currently unknown other than that Bill Trinen calls them "talented" and "new" . In short, we basically just know it's not Retro Studios. So here's what we know in total so far: it's a Metroid game, it was announced during E3 2017 and Retro Studios isn't handling it. Now we can add vague timeline info into that equation.
Speaking to Mashable, Nintendo of America President Reggie Fils-Aimé stated that they already have "expectations" as to when Metroid Prime 4will be released, and he's able to make that claim because "the game is well into development." Well, eh? Well...that's great. It might be years off and get that "Nintendo" delay for the sake of quality, but it's good to know that progress is being made.
As for announcements in general, Reggie says it "depends on the game," and "how we feel the consumer is going to respond to the message." Ha! That almost feels like a straight reference to Diablo Immortal, though Nintendo is no stranger to Metroid Prime: Federation Force style reactions. It was definitely the right time for a general "a mainline Metroidis coming" message after folks had been pining for it for years.