What is the term for a male horse that is under the age of four?

The term for a male horse under the age of four is "colt." This designation is specifically used to refer to young male horses that are not yet sexually mature. Generally, colts are considered to be horses between the ages of birth and about three years old.

Understanding the definitions of other relevant terms can provide clarity: a foal refers to any young horse, male or female, that is less than one year old; a yearling is a horse that is between one and two years of age; and a stud typically refers to a male horse that is used for breeding. Thus, the term "colt" accurately captures the characteristics and age range of a male horse under four years of age.