A rational number can be written in the form of $\frac{p}{q}$ where $p$ and $q$ are integers and $q$ is not equal to zero. Rational numbers have their decimal expansions as either terminating or non-terminating but recurring. 
$\pi$ = 3.14159265358..., i.e., the digits after decimal point do not terminate and not recurring. Hence it is not rational
$\sqrt{2}$ is approximated to 1.414213562373095..., i.e., it cannot be expressed as a recurring and non-terminating decimal and hence is not rational.
$\frac{2}{0}$ is not defined and for a rational number, the denominator should be non-zero.
4.8 on the other hand, satisfies all the properties of rational numbers.