4. ΔABC\Delta ABCΔABC的内角AAA，BBB，CCC的对边分别为aaa，bbb，ccc．已知asinA−bsinB=4csinCa\sin A-b\sin B=4c\sin CasinA−bsinB=4csinC，cosA=−14\cos A=-\frac{1}{4}cosA=−41，则bc=(\frac{b}{c}=(cb=( )))