Question

What does a tetradic color scheme mean?

Answer 1

"Rectangle (tetradic) color scheme
The rectangle or tetradic color scheme uses four colors arranged into two complementary pairs.
This rich color scheme offers plenty of possibilities for variation.
Tetradic color schemes works best if you let one color be dominant.
You should also pay attention to the balance between warm and cool colors in your design."

Source: http://www.tigercolor.com/color-lab/color-theory/color-theory-intro.htm#rectangle