Smash for Wii U is a fine game that brought me tons of great memories and introduced some of my favorite characters to the series (Little Mac!), but stands as the only Smash game that I have no reason to ever revisit.
Keep in mind I'm rating in retrospect, but I've put hundreds of hours into this game and loved it. This may be an unfair metric to rate with, since Ultimate was just a blip on the radar at the time, but time hasn't been too kind to Smash 4 since Smash Ultimate improves on everything established by this game tenfold. The single player options are not exceptionally good and the game doesn't have a particularly distinct game feel or atmosphere either - draining Smash of some of its unique identity that was so great in the games prior, but I digress. The replay value is nonexistent.
The moveset design shows its age a bit too. Smash 4 relied very heavily on creating gimmicky characters to keep the roster fresh, but in the process created some occasionally clunky or outright broken characters (...Little Mac). The best designs were probably the third party characters, Mega Man and Pac-Man are exceptional and it's hard to imagine Smash without them. Definitely an experimental period for character design that would eventually lead to Ultimate's more polished approach that has created some of the most fun characters in the series.
So in short, I guess Smash 4 is kind of just a stepping stone that leads us from Brawl to Ultimate. It doesn't do much to distinguish itself beyond that, but it was a fun game at the time and the improvements from Brawl to Smash 4 have been valuable ones. And I mean... it's Smash Bros.
Keep in mind I'm rating in retrospect, but I've put hundreds of hours into this game and loved it. This may be an unfair metric to rate with, since Ultimate was just a blip on the radar at the time, but time hasn't been too kind to Smash 4 since Smash Ultimate improves on everything established by this game tenfold. The single player options are not exceptionally good and the game doesn't have a particularly distinct game feel or atmosphere either - draining Smash of some of its unique identity that was so great in the games prior, but I digress. The replay value is nonexistent.
The moveset design shows its age a bit too. Smash 4 relied very heavily on creating gimmicky characters to keep the roster fresh, but in the process created some occasionally clunky or outright broken characters (...Little Mac). The best designs were probably the third party characters, Mega Man and Pac-Man are exceptional and it's hard to imagine Smash without them. Definitely an experimental period for character design that would eventually lead to Ultimate's more polished approach that has created some of the most fun characters in the series.
So in short, I guess Smash 4 is kind of just a stepping stone that leads us from Brawl to Ultimate. It doesn't do much to distinguish itself beyond that, but it was a fun game at the time and the improvements from Brawl to Smash 4 have been valuable ones. And I mean... it's Smash Bros.