So, I've been comparing the dates for Brawl's development and make a prediction for SSB4's release date, assuming that SSB4 takes the same amount of time.
Brawl's development started in October 2005, giving us November and December as two months of development. (2)
As the game was only released in January 2008 in Japan, that gives us two entire years of development time. (24 + 2 = 26)
Doing the math, we have 26 full months of development time.
Now that we have a release date for 2014, we can do the same thing for SSB4.
Development started in February 2012. From March to December, we have 10 months of development. (10)
Since the game will only be released in 2014, that means 2013 is a full year of development. (10 + 12 = 22)
By the end of 2013, the game should be at 22 months of development. If Sakurai is given the exact amount of time as Brawl, development should end in April 2013, giving us a possible release in May or June of the next year.
However, it's very possible that the game take more time to be finished and let's not forget that there's a bigger development team this time. I believe the game will be released on the summer of 2014 at best and Christmas 2014 at worse.