This information comes from Nintendo of America president Reggie Fils-Aime…

“You know, for us, we believe that having hands-on opportunity married to an announcement is really the best way to do it. And so let’s take Super Mario Odyssey for example. We could have announced it months ago, but we weren’t yet ready, the team wasn’t ready, to show it and to let the consumer really understand visually how the hat mechanic works, how the capture mechanic comes into play. And so that’s how we think these through.”

“For certain games, games that will be in development for, let’s call it a decent amount of time, like Metroid Prime 4 — also, given that it’s a franchise that we know people have been very eager to get some news — that’s when, fine, we’ll share it. We’ll share it early. Others, we want to hold closer in and reveal it when the gameplay is going to be available. It literally is game by game, title by title, how we make that decision.”

Source

SMTXFE or Tokyo Mirage Sessions was done like this. Shown before development began.

that sounds like MP4 is a 2019 or later title

Good decision. Because that saved e3!

Now please also announce Animal Crossing for the Switch!

I remember a while back when Nintendo took a shot at Sony for announcing stuff years in advanced.

Hopefully they don't take too long, the Metroid fans may be appeased for now, but if they delay too long they'll get restless again!

let the speculation begin on what a "decent amount of time" translates into...

^ ;)

They could have saved this announcement for when they actually had something to show.
Fans have been waiting for so long... they could wait a little bit more.

I get it. They probably weren't planning on doing MP4 but then demand became so loud that they couldn't ignore it and started developing it. But it will probably take 2-3 years to make from here and they don't want fans to just think they have been ignoring the increasing demand so they do a little tease like this.

I'd imagine they will announce that in the next few directs. Probably a Japanese direct first because the Japanese LOVE Animal Crossing and E3 isn't the best place to reveal a game that is mainly popular there (I too love AC btw).