4 can be expressed as a fraction by placing it over 1. Therefore, as a fraction, it is written as 4/1.
Since both the numerator (4) and the denominator (1) have no common factors other than 1, this fraction is already in its simplest form. Any whole number can be converted to a fraction by using 1 as the denominator, and in this case, the simplest form of 4 as a fraction is 4/1.