4.
如图,在三棱柱\(ABC\)\(\)\(A\)\({\,\!}_{1}\)\(B\)\({\,\!}_{1}\)\(C\)\({\,\!}_{1}\)中,\(AA\)\({\,\!}_{1}\)\(C\)\({\,\!}_{1}\)\(C\)是边长为\(4\)的正方形,平面\(ABC\)\(⊥\)平面\(AA\)\({\,\!}_{1}\)\(C\)\({\,\!}_{1}\)\(C\),\(AB\)\(=3\),\(BC\)\(=5\).
\((1)\)求证:\(AA\)\({\,\!}_{1}⊥\)平面\(ABC\);
\((2)\)求二面角\(A\)\({\,\!}_{1}\)\(BC\)\({\,\!}_{1}\)\(B\)\({\,\!}_{1}\)的余弦值.