Shade IV is described as a "sheer ruby." It's a muted, medium-dark red with subtle, warm undertones and a very fine pink-red shimmer. It's more semi-opaque than sheer and once on, actually has more of a faded, muted quality to it than when swatched. It looks almost pink-red applied to me. It lasted for four hours on me. See comparison swatches / view dupes.
FURTHER READING: Formula Overview, which is a detailed review of the range as a whole.