To simplify the left side of the equation, we will first expand the expression:

(sin^2x - cos^2x)^2 - 4cos^2xsin^2x
(sin^2x - cos^2x)(sin^2x - cos^2x) - 4cos^2xsin^2x
(sin^4x - 2sin^2xcos^2x + cos^4x) - 4cos^2xsin^2x

Next, we will use the trigonometric identity sin^2x + cos^2x = 1 to simplify the expression:

(sin^4x - 2sin^2xcos^2x + cos^4x) - 4cos^2xsin^2x
(sin^4x - 2sin^2xcos^2x + cos^4x) - 4sin^2x(1 - sin^2x)
sin^4x - 2sin^2xcos^2x + cos^4x - 4sin^2x + 4sin^4x
5sin^4x - 2sin^2xcos^2x - 4sin^2x + cos^4x

Therefore, the simplified expression of (sin^2x - cos^2x)^2 - 4cos^2xsin^2x is 5sin^4x - 2sin^2xcos^2x - 4sin^2x + cos^4x.

It seems that the given result of -0.5 is incorrect based on the simplification we have done. The actual result may be different or there could have been an error in the initial equation.