About Prime 4 and lack of Phazon - that is totally a thing, but there are still some unresolved bits they were throwing around that can still be expanded on. Like for one example, what the heck was Sylux doing in the endings of multiple games. If it was only one game, I could see some people thinking it was a one-off thing, but instead it seems more like they were deliberately setting something up. Another example, though a bit more questionable (it could have been an oversight) is that Phazon was still present in Samus's arm cannon in the ending sequence of Corruption, after the destruction of Phaaze and the disappearance of Phazon elsewhere. This may or may not be a thing that was done intentionally to lead into something else.
And there's a bit of a disagreement on this in the Metroid community, but in my honest opinion, I feel like Hunters was a good Prime game in its own way, despite being rather different than the others. If it wasn't more focused on multiplayer, I feel that it could have been more fleshed out and perhaps fit in better with the others. But as it were, the original Prime system does not make for very good or long-lived multiplayer, as seen in Echoes - and Hunters did not end up making for a particularly memorable singleplayer. So I guess it was one or the other, and they went for multiplayer focus.
Anyways, what I'm getting at is that there was no Phazon present in Hunters either, and Sylux is from said game. Considering he has made appearances in multiple endings now, it's possible they may have something in mind that does not directly involve Phazon. What this means for Prime 4, I don't know. But no matter what's the case, I'm really looking forward to this one.