But directly following the initial division, movement and inertia were diametric opposites, opposing forces. Combining them requires common ground. It could not be done directly.
So there are two energetic go-betweens, the transitionals. Comprised of both movement and beingness they transition between the two opposites to create the full cycle.
The year with its four seasons is another way The Universal Cycle is expressed by the physical world we see around us. Again, if we observe the underlying dynamic in terms of our molecular model, applying heat causes movement, creating the full bloom of summer. While winter's cold causes inertia to the point that everything becomes frozen still.
Spring and fall, transitioning us from one extreme to the other, are seasons in their own right. They each contain both winter's cold, as well as summer's heat and are expressions of the quaternal order that underlies the Universe.