If any of the rumors are true, it will most likely be a port, in which case Smash 4 would die instantly due to being inferior to a version on a more popular console. If it is Melee HD, I think some of the Smash 4 community and most of the Melee community will probably make the switch (pun intended). If it is a new game altogether, it's hard to say. It depends on how good it is competitively.
--WARNING-- Minor unnecessary rant that explains my last sentence.
I believe that the only reason the Smash community is so divided between Melee and Smash 4 is because Brawl was not a good competitive game. If Brawl were good, I'm sure most of the Melee community would have switched, as opposed to just some (I know some still would have stayed, like with 64, but that's irrelevant). If that were the case, then the community as a whole would have gone from Melee->Brawl->4. Instead, what happened is that Brawl was bad, some players switched because it was new, others didn't because it was bad. Those who switched carried on to 4, but those who stayed had by that point already invested too much in Melee to give it up, thus creating a divide between the people who played the newest and the people who played the "best".