The "4Cs" refer to four essential factors that determine the quality and value of a diamond: cut, color, clarity, and carat weight. The cut affects how well the diamond reflects light, while color measures the presence of any hue in the diamond, with less color being more desirable. Clarity assesses the presence of internal or external flaws, known as inclusions and blemishes, respectively. Lastly, carat weight indicates the size of the diamond, with one carat equaling 200 milligrams. Together, these factors help consumers evaluate and compare diamonds effectively.