A 4-carat diamond ring is a ring whose diamond or diamonds add up to a total of 4 carats. The ring can have a solitary diamond, a larger diamond surrounded by smaller diamonds, or a grouping of diamonds that are all the same size but add up to 4 carats.
A carat is a measurement of how much a diamond weighs. It is one of the ways that the quality of the diamond is assessed. The others are clarity, cut and color.
A carat weighs 200 milligrams, so the diamond or diamonds in a 4-carat diamond ring would weigh 800 milligrams.