If sin4a + 4 cos4b + 2 = 4 2 sin a cos b ; a, bÎ[0, p], then cos(a + b) – cos(a – b) is equal to :

If $\sin ^{4} \alpha+4 \cos ^{4} \beta+2=4 \sqrt{2} \sin \alpha \cos \beta ; \alpha, \beta \in[0, \pi]$, then $\cos (\alpha+\beta)-\cos (\alpha-\beta)$ is equal to: