HAL's....not actually working on smash 4. They didn't work on Brawl either. Their company name is only on brawl because they own the name.
Even if they were, Hal has at least three separate teams at any given time - two of which work on games and the third of which works on using new hardware in games. For example, back on the N64, there was a team working on smash, one working on K64 and a third working on implementing gyroscope technology, which led to tilt and tumble. Hal's also the guys who made the e-reader, actually. Also the ones who made the original DS drawing engine used in games like canvas curse and pokemon rangers. And they made the camera/gyroscope combination on the 3ds, which is what face raiders was for.
For now, we've seen three games by three teams released around the same time (KMA, KRtDL, Face Raiders). Helping out with another smash on top of doing other stuff would not be difficult for them. Add that Iwata's making a Kirby-related announcement tonight and another Kirby game (or at least a remake) seems pretty likely.