The lights are on
Ugh, people need to stop saying "Metroid Prime 4." There is little chance of a "Metroid Prime 4." The Prime series was a trilogy taking place between Metroid 1 and Metroid II, and with the death of Dark Samus, which was a creature formed from the lifeform known as Metroid Prime, the series is over. It was a trilogy built around Phazon, the Metroid Prime, and Dark Samus.
Hopefully, Retro is making a brand new Metroid game and, while I doubt it will happen, I'd prefer them to make a game erasing Other M from the continuity. If nothing else, I'd be happy to see Retro given free reign to make whatever the hell they want.