The four primary colors in the 4-primary color wheel are blue, yellow, green and red. This differs from the color mixing wheel, which only has three primary colors.
Calling the colors "psychological primaries", Ewald Hering, a German psychologist, came up with the concept of the 4-primary color wheel in 1878. It is used within the commercial color and paint industries, and is a great way to understand and teach the differences between warm and cool colors.
Leonardo da Vinci recognized these four simple colors well before Hering's time. The only distinction being that Hering organized the colors into a circular model so that the colors and their combinations could be more easily observed. He then used the color wheel to indicate the amounts of warm and cool hues that have gone into creating the different colors that are shown, making it a handy visual reference for seeing how much of each color is needed to achieve a certain shade.
It shows also how each of the primary colors has both a warm and cool side, and shows which colors are complementary to each other, as well as opposing each other. Complementary ones mix well together, while opposing colors typically do not blend together as well and should be avoided.