What prefix indicates the number 'four'?

The prefix that indicates the number 'four' is "tetra-." This Greek-derived prefix is commonly used in various scientific and medical terms. For example, tetrahedral refers to a shape with four faces, and tetralogy denotes a set of four related medical conditions. "Quadri-" is also a prefix associated with 'four,' but it is more often used in specific contexts such as quadriplegia, which pertains to paralysis of all four limbs.

The confusion may arise from the close meanings of "quadri-" and "tetra-," as they both can refer to four. However, "tetra-" is the more traditional prefix derived from the Greek language, whereas "quadri-" comes from Latin, and their usage can vary based on the specific terminology.

In summary, "tetra-" is the preferred prefix in many scientific and medical contexts to indicate the number four.