What is the least common factor of 4 and 5?

The least common factor is a term often mistakenly given to either the greatest common factor (GCF) or the least common multiple (LCM). This term is not often used because it does not describe a useful relationship between numbers. Since 1 evenly divides all integers, 1 is technically the least common factor for any set of integers.

4 and 5 are relatively prime, which means that they share no factors aside from 1.
The least common factor of any set of positive integers is 1.