What is the standard measurement used for horses and ponies, equal to four inches?

The standard measurement used for horses and ponies is referred to as a "hand." This term is a traditional unit of measure that is specifically defined to be equal to four inches. The use of "hands" allows for a standardized way to measure the height of horses and ponies, which is typically expressed in hands and inches (for example, a horse that is 15 hands and 2 inches tall would be written as 15.2 hands).

The measurement is rooted in historical practices where a person's hand was used to gauge size, hence the name. The hand measurement system is widely accepted in the equine community, making it an essential part of horse evaluation, breeding, and classification.

In contrast, a foot measures twelve inches, an inch is a smaller unit of measure, and a yard equates to three feet, all of which are not suitable for or specific to the context of measuring equine height. Therefore, "hand" stands out as the necessary choice for measuring horses and ponies.