If the function $f(x) = \left\{ {\begin{array}{*{20}{l}} \begin{array}{l} \frac{2}{x}\left\{ {\sin \left( {{k_1} + 1} \right)x + \sin \left( {{k_2} - 1} \right)x} \right\},\quad x < 0\\ 4,\quad x = 0\\ \frac{2}{x}{\log _e}\left( {\frac{{2 + {k_1}x}}{{2 + {k_2}x}}} \right),\quad x > 0 \end{array} \end{array}} \right.$ is continuous at x = 0, then ${\rm{k}}_1^2 + {\rm{k}}_2^2$ is equal to