The number .4 is written as a what?

Decimal

Step-by-step explanation:

A dot means decimal.

In the question you are asking, there's a dot. That means decimal.

Decimal meaning:

A fraction whose denominator is a power of ten and whose numerator is expressed by figures placed to the right of a decimal point.

Be careful of this:

, is used when you have a big number its easier to understand if you have a comma.

. is used when it's so close to a number but, not quite all the way to the whole number.

The comma is sometimes used as "." be very careful about this mistake.