We have the number:

We have to answer if it is rational or irrational.
In order to be rational, a number has to be able to be represented in a fraction.
In this case, we start by separating the number in the whole part and the decimal part:

We can express the decimal part as a fraction, because:

Then, we have:

As we have a fractional expresion for the number, it is not an irrational number.
4.3... is a rational number.