Also, it looks like 3 other cores merge with 10% when it changes formes, which somewhat confirms that Zygarde will likely have something to do with the Sun/Moon legendaries. Four cores likely means a core for Xerneas, Yveltal, Solgaleo, and Lunala. It also coincides with the four snake head things that come out of Zygarde's shoulders.
That seems to just be aesthetics of the form change as there are actually four red spots that appear in the transformation, and said spots are the same color unlike the supposed cores.