The decimal number 0.4 is equivalent to 4/10 in fraction form. This fraction can be reduced to its simplest form by dividing both the numerator and the denominator by their greatest common factor (GCF). The GCF of four and 10 is two. Therefore, 0.4 is equal to 2/5.

According to Math is Fun, one method of converting decimals into fractions is to write down the decimal divided by one (0.4/1). Multiply both the numerator and denominator by 10 for every number after the decimal point. Since there is one number after the decimal point, multiply both the numerator and denominator by 10. The numerator will be four (0.4 x 10), and the denominator will be 10 (1 x 10). The last step is to reduce the fraction into its simplest form (2/5).