Which prefix represents the number "four"?

The prefix that represents the number "four" is "tetra-." This prefix is derived from the Greek word "tetra," which directly translates to four. It is commonly used in various medical and scientific terms to denote groups or structures comprising four elements. For example, in medical contexts, "tetralogy" refers to a set of four abnormalities, often used to describe specific congenital heart defects known as tetralogy of Fallot.

The other prefixes do not denote the number four; "tri-" refers to three, "hemi-" indicates half, and "penta-" signifies five. This understanding of numerical prefixes is essential in medical terminology, as it aids in the accurate description of conditions, structures, and measurements within the healthcare field.