如图，反比例函数$y=\frac{k}{x}(x>0)$经过$A$、$B$两点，过点$A$作$\mathit{AC}\perp y$轴于点$C$，过点$B$作$\mathit{BD}\perp y$轴于点$D$，过点$B$作$\mathit{BE}\perp x$轴于点$E$，连结$\mathit{AD}$，已知$\mathit{AC}=1$、$\mathit{BE}=1$、$S_{\mathit{矩形}\mathit{BDOE}}=4$．则$S_{\mathit{\Delta ACD}}=$　　．