They've already been developement for almost 5 months now with another 4 months to go & using the Brawl engine as a building block means that the 2013 date may be possible, maybe won't be until late 2013, like a Holiday Release
Late 2013 is unlikely, but early-mid 2013 is ridiculous. They won't have the game done with 4 more months of development time. We'll be lucky to get a trailer within the next four months.
Plus this isn't the first Smash to use a previous Smash as a "building block". Brawl used Melee and that still needed nearly 3 years to complete.
You don't think Project Sora & Namco together can finish the game in a years & a half's time?
I think technically it might be possible, but knowing Sakurai's demand for polish and inclusion of a high amount of content, plus the fact that two games are being developed, I'd say it would be extremely unlikely. If they did make the game in a year and a half's time, it would be very light on content, depth, and polish compared to previous titles and fan expectations.
We're talking about a company who has created countless fighter titles in that exact time frame of each other, & we're talking about a team who isn't just making a new game or a sequel but the fourth game in the series, they are skilled at these types of things.
Firstly, most Namco fighters do in fact take more than a year and a half of development. The few that don't aren't necessarily heavy on content, even for a fighting game (not that I'm saying they're bad games).
Secondly, Sakurai has used a different team for each Smash development since Melee, and the team(s) that worked on Brawl is not the same team that will be working on this one. Since Project Sora closed, I assume development will mostly be handled by Namco, who obviously never worked on Smash before.
I expect a new & polished Smash Bros, but nothing innovative that will change fighting games or Smash Bros. forever, so why would it be something as time consuming as 3 years, I'll give you 2 years okay, but I believe this will be no serious time consuming thing
Even if nothing new is implemented (though we all know something will be, Sakurai has already mentioned customization and connectivity) polish alone will require a fair amount of time, considering how much content Sakurai includes in each Smash. Even if we do somehow get the game with 2 or less years of development, that would be quite the unlikely situation, so I wouldn't get my hopes up for a release before Q3/Q4 2014.
I remember that Sakurai said that he wanted to make something different from previous titles, and bearing in mind that will gonna be a portable and a HD version, i cannot imagine the same Brawl source code in use for these projects.
5 months? SSB4 development started in second half of June. He didn't immediately started production after finishing KIU. I'm expecting that Smash Bros 4 will be released on holidays of 2014 or first half 2015.
KIU finished development in January IIRC, and Sakurai only took about a month off; Smash 4 development began (at least on Sakurai's part) in late February, not June. Your release prediction is probably pretty likely though.
And just because the Brawl engine is being used, doesn't mean it will be the same as Brawl. As you said yourself, Brawl used Melee's, and everybody knows how different they were.