sin^4 x = sin^2 x * sin^2 x
sin^4 x = (1 - cos^2 x)(1 - cos^2 x)
sin^4 x = (1 - cos x)(1 + cos x)(1 - cos x)(1 + cos x)
sin^4 x = (1 - cos x)2 * (1 + cos x)^2
Either of the last two equivalent expressions will work, as they both only involve cosine to the first power.
