Let's look at the facts:
The first Smash was released in 1999, Melee in 2001, Brawl in 2008, and Smash 4 in 2014. Aside from between Melee and 64 (which, given how small 64 was, made sense), there seem to be around 6-7 years between each Smash games. There have been rumors about a Smash game being released with the NX in 2017. Smash 4 being out for less than 3 years before being replaced? That doesn't seem very likely, especially considering how huge Smash 4 is.
Let's look at a few other factors. Third party characters. Smash 4 has Bayonetta, Cloud, Sonic, Ryu, Mega Man, and Pac-Man, not counting assist trophies. Granted, some (or all) of those could return for the next instalment, but chances are Nintendo would have to make some new deals then. I don't think they are ready to do that just yet. Let's not forget about all the music and trophies from third party franchises as well.
There is also the Sakurai factor. Would Sakurai release a whole new game in less than three years? I highly doubt that. Granted, they could make a brand new Smash game without Sakurai, but why rush it?
So, I think one of two things will happen: Either the next Smash game will be "Super Smash Bros. for Nintendo NX" (or whatever the console is called), which would be an enhanced remake of Smash 4 (similar how the Wii U is to the DS version), or it won't come until 2019 or 2020 (potentially late 2018).
Oh, and if there is no new game, then I don't see any reason for why Smash 4 would die. Melee is still around, after all.