According to Eurogamer Bandai Namco Studios Singapore is working on Prime 4.
Source
Rumors about Namco being behind Prime 4 have been going around for months so I can see this being the case and it seems like they play a part in almost every Nintendo game these days.
Eurogamer wrote:Internet rumours this week pegged Bandai Namco as being the mystery studio behind Metroid Prime 4 - something multiple sources have now confirmed to Eurogamer as being accurate.
Specifically, the highly-anticipated Nintendo Switch exclusive is being developed by Bandai Namco Studios Singapore, which we understand is lead studio on the game in collaboration with Nintendo.
This studio houses some former LucasArts Singapore staff who previously worked on, among other things, the ambitious cancelled Star Wars 1313.
Metroid Prime 4 was briefly announced via a logo shown during Nintendo's E3 2017 broadcast. During a media pre-briefing, journalists asked Nintendo exactly who was behind the game. Nintendo declined to reply, except to say that Retro Studios, which created the original Metroid Prime Trilogy, was not behind the project.
Since then, speculation has been rife over who was working on Metroid Prime 4, but nothing more has been announced about the game since.
It is still relatively early in the game's development - there's no suggestion Prime 4 will launch this year (which, perhaps, might just leave room for a port of the Metroid Prime Trilogy compilation to get Switch owners up to speed on the story so far).
Source
Rumors about Namco being behind Prime 4 have been going around for months so I can see this being the case and it seems like they play a part in almost every Nintendo game these days.