Consider the expression $f(x)=\sqrt[\phantom{2}]{4-x^2}.$

Q$\mathrm{.}2\mathrm{.}4\mathrm{.}1$ Determine its domain.

Q$\mathrm{.}2\mathrm{.}4\mathrm{.}2$ Briefly explain the relevance of the domain of this function in the context of input validation in programming.