Researchers from AU Optronics developed an AMOLED display that uses four primary colors (Red, Green, Blue and Yellow). They say that this design is more efficient than an RGB panel by 16%. The AMOLED panel is 4.65" 720p (317 PPI).
RGBY has been used by Sharp for their Quattron LCD displays (Aquos TVs) for a long time. Sharp says that adding another color increases the range of displayable colors (although the content itself does not provide the yellow color information, which means the TVs need to use software processes which results in less accurate colors).