Nếu \(\int\limits_0^1 {xf\left( x \right)d{\rm{x}}} = 4\) thì \(\int\limits_0^{\frac{\pi }{4}} {f\left( {{\rm{cos}}2{\rm{x}}} \right)} \sin 4{\rm{xdx}}\) bằng: A. 2 B. 6 C. 8 D. 4