I believe the two factions of the Nine are simply the terrestial planets and the jovian planets. Numerically, it would match up, with the 4 being the terrestial planets (mercury, venus, earth and mars) and the 5 are the jovian planets (jupiter, saturn, neptune, uranus, and the mysterious, theoretical planet 9). Because of the differences between a rocky and a gaseous surface, it would make sense that the planets might have a conflict of interest if they were sentient beings. Also, i think that the 4 might be more determined to get rid of their dependence on life because life has been the most dense on the 4 rather than the 5. Earth was the first one to actually have life, and when the traveler came to the sol system, it can reasonably be assumed that mercury, venus, and mars were the most densely populated planets because they are located close to the mother planet, Earth. When human lifespan tripled, there was a huge boost in population, so the 4 would have more life which equates to more dependence on that life. Therefore the 4 will have a greater conviction to get rid of the dependence than the 5. Now the 5, in comparison, would be more interested in the Light because they have not experienced as much dependence as the 4. They also witnessed firsthand the power of the Deep, so the 5 will have different motives than the 4.