applying the double angle formula again to sin(4x), you will get 2sin(2x)cos(2x). You can see that you already have sin(2x)cos(2x). so if you do what you just did before and you will get 1/4 sin(4x)
edit: its just the double angle formula rearranged;
sin(2x)=2sin(x)cos(x)
1/2*sin(2x)=sin(x)cos(x)