Why did Nintendo choose to make a new numerical sequel for the series? Numbers too big look silly for games that aren't Final fantasy. They don't do it for Mario or Zelda so why MP? Or is this name just a place-holder to make it more clear to the public on what type of Metroid game they're working on, and later-on will be replaced with an undertitle?

Metroid Prime is a series, Mario and Zelda are a franchise. When Nintendo has a series of Mario games they do number them - Super Mario Bros 1, 2, 3; Super Mario Galaxy 1 and 2; New Super Mario Bros 1 and 2. 

They want to make clear it's a sequel, not a remake or a reboot.

Basically what Jumpin said. Notice how SMB was numbered? They number games that are a part of a series.

Metroid Prime 9

You see how silly it looks?

Thought this was going to be a Niceguygameplayer thread

"Why is Metroid Prime 4 on Switch? Switch is for kiddie games. Nintendo should develop it for Playstation 4!"

But there's no point for making other games  than fps (Prime) with the franchise. They have enough platformers so that's how this series is going to look like for the next 10 years.

Silly?Countless franchises do that, and they are currently in front of Prime in the numbers front.

But I personally believe its just a placeholder, something meant to signal that yes, its a metroid game and yes, its in the vein of the Prime games( in term of atmosphere and gameplay direction) and they have done it so that they could get that across in a 15 secons video without showing footage, but Im 99,99999999% sure that they will either drop the number and use a subtitle or just rename it as Metroid Something.

Uncharted 4 is a silly name. Got it.

There won't be a 5 because Sony feels what I do.