“I shouldn’t say, it’s a little tricky… To be completely honest, at this E3 I just want people to pay attention to Luigi. If I mention something about Metroid Prime 4, people will focus on that. But when the time comes, I am so ready to talk about it!” ~Tanabe on not having Metroid Prime 4 at E3.
Nice to have an explanation as to why Metroid Prime 4 wasn't there, and the decision might end up helping Luigi's Mansion 3 in the long run.