$\left\{ \begin{matrix} {{Q}_{1}}=C{{V}_{1}}  \\ {{Q}_{2}}=C{{V}_{2}}  \\ \end{matrix}\Rightarrow {{Q}_{2}}-{{Q}_{1}}=C({{V}_{2}}-{{V}_{1}})\Rightarrow \Delta Q=C\Delta V\Rightarrow 10=C\times (6-4)\Rightarrow C=5\mu F \right.$ $U=\frac{1}{2}C{{V}^{2}}=\frac{1}{2}\times 5\times {{6}^{2}}=90\mu J$