Tính S4?

\[{S_4} = \frac{1}{2} + \frac{1}{3} + \frac{1}{4} + \frac{1}{5}\]

\[{S_4} = \frac{5}{6} + \frac{9}{{20}}\]

\[{S_4} = \frac{{100 + 54}}{{120}} = \frac{{154}}{{120}} = \frac{{77}}{{60}}\]