What is sin(4x) in terms of C if cos(2x) = C

This is where I've gotten so far:

 

 

$sin(4x) = 2sin(2x)cos(2x) = 2Csin(2x)$

 

I've then tried using the fact that sin(2x) = cos(2x-pi/2) to simplify but that didn't help me since I was left with a sin(2x) term anyway. I couldn't find an identity that would let me rewrite sin(2x) in terms of cos(2x)'s. 

 

Thank you.