Find the value of x such that : \({{\rm{x}}^{{{\rm{a}}^2}}}.{{\rm{x}}^{{{\rm{b}}^2}}}.{{\rm{x}}^{{\rm{ab}}}}.{{\rm{x}}^{{\rm{ab}}}}{\rm{\;}} = {\rm{\;}}{\left( {{3^4}} \right)^4}\) and (a + b) = 4.