If the game is coming out anytime this year all the characters probably would've been finished conceptually by this point. I very, very much doubt they'd make one character at a time.
Technically they do if you are talking about one team. I don't have any experience at any major, but I learned from what I heard. They actually focus on a character at a time depend on which teams that are working on. I'm terrible with explaining but I will use an excuse instead. However they might work differently than other companies, so don't take my words or examples as 100% fact.
Art & Model Team would work on a new character model (Takamaru) and they just completed their job with veteran character. (Fox) So they would pass it over to other team to work on character and install them into the game.
Moves & Behavior Team would receive the model of Fox from other team, and port Brawl's files into SSB4. Of course, they would adjust the files and make it much better. Hopefully. Once they are done with setting up the moves, and giving them facial expression and behaviors (Stand By, Taunts, etc) (I'm not sure if Moves, Behavior, etc would be in a same team, but it's just an example.)
HitBox + Balance Team
You know how it work. blah blah blah.
Once they are done with a bunch of works, then they probably would have beta testers to testing the games out. (usually at late phrase of development.) Technically if they only have three teams then they could work on three characters at once with three different teams.
Edited: Also those teams are for character progress only, and I'm pretty sure that they had other teams working on the stages, trophies, etc.