4.
已知公比小于\(1\)的等比数列\(\left\{ {{a}_{n}} \right\}\)的前\(n\)项和为\({{S}_{n}}\),\({{a}_{1}}=\dfrac{1}{2}\),\(7{{a}_{2}}=2{{S}_{3}}\).
\((1)\)求数列\(\left\{ {{a}_{n}} \right\}\)的通项公式;
\((2)\)设\({b}_{n}={\log }_{2}(1−{S}_{n+1}) \),若\(\dfrac{1}{{{b}_{1}}{{b}_{3}}}+\dfrac{1}{{{b}_{3}}{{b}_{5}}}+\ldots +\dfrac{1}{{{b}_{2n-1}}{{b}_{2n+1}}}=\dfrac{5}{21}\),求\(n\) .