Yin-Yang represents the ancient Chinese understanding of how things work. The outer circle represents "everything", while the colored shapes within the circle represent the interaction of two energies, called "yin" (the black or darker color) and "yang" (the white or lighter color). The two are responsible for everything that happens.
But each half of the circle is not completely dark or light, just as things in life are not completely black or white, and they cannot exist without each other.
These "balanced" designs fit your 4" x 4" hoop and a pdf-formatted thread chart for each design is included.