It might have been Sakamoto but during the Wii U era he talked about a new Prime game but said if they were to make one it would be the next platform (Switch) I think that interview was 2014 or 2015?

Development for Prime 4 didn’t start until 2016 at the earliest. Its reveal in 2017 was solely intended to respond to distraught fans who were overwhelmingly opposed to the 3DS spinoff entry.

Looking back, it's probably a good thing they did announce it in 2017, otherwise I expect when it went off the rails they may well have just cut their losses and cancelled it altogether.

Probably the only reason they remained committed to it is because they'd already announced it and so going back on that would cause them to lose face.

I cross-referenced the development credits, and there are 23 people on the Prime 4 team that worked on previous Prime titles:

President:
Michael Kelbaugh

Studio Operations:
Ryan Harris (Prime 2)
Al Artus (Prime 1)

Designers:
Russell O'Henly (Prime 2)
Bill Vandervoort (Prime 3)
Jay Fuller (Prime 3)
Jonathan Delange (Prime 3)

Engineers:
Alex Quinones (Prime 1)
Jim Gage (Prime 1)
Ryan Cornelius (Prime 3)
Andy Hanson (Prime 3)
Jesse Spears (Prime 3)
Aaron Walker (Prime 3)

Artists:
Ryan Powell (Prime 1)
Teague Schultz (Prime 2)
Sean Horton (Prime 2)
Matt Manchester (Prime 3)
Thomas Robins (Prime 3)

Animators:
Dax Pallotta (Prime 1)
Stephen Zafros (Prime 1)
Rapheal Perkins (Prime 3)
Will Bate (Prime 3)

Audio:
Scott Petersen (Prime 2)

Yeah I know. That interview which I don't remember the source of was talking a IF scenero. IF they were to make a new prime it would not be on Wii U is what he was confirming not that a new Prime was being made.

I thought it was still a pretty good game despite its flaws.

If it turned out THAT good despite the evident development hell it went through, that is extremely encouraging and makes me excited for a Metroid Prime 5 that is entirely, 100% made by Retro Studios with their own original vision and design.

...Now I just hope Prime 4 sells well enough to justify a Prime 5.

I'd like the desert sections if there was more going on out there, and I'd also like if it was scarier like Prime 2 and Dread. Thought fusing with Metroids would result in certain characters displaying more body horror, like at least close to David Cronenberg's The Fly.

There's only so much body horror you can pull off while retaining a Teen rating; Metroid is niche enough without limiting its audience even further with an adult rating.

I know, but so far I haven't found anything as scary as, say, Draygon from Super Metroid or Nightmare from Metroid Fusion.

Yes it's upsetting and disturbing when you realize Carvex's situation after killing it, but would've been scarier if it tried to communicate with what little humanity (in a sense) it had left post Metroid infection, saying something like "Help me...".

I mean the Prime series has never been as horror-oriented as some of the stuff in the 2D series.

Prime 1-3 had their moments of darkness like the experiments on the Orpheon in Prime 1 or the GFS Olympus in Prime 3, but they were always a bit more understated, Prime 4 simply continues with this trend.