Let \( f(x) = \sin^{4}x + \cos^{4}x \). Then f is an increasing function in the interval:
1
\( \left [\dfrac {5\pi}{8}, \dfrac {3\pi}{4}\right] \)
2
\( \left [\dfrac {\pi}{2}, \dfrac {5\pi}{8}\right ] \)
3
\( \left [\dfrac {\pi}{4}, \dfrac {\pi}{2}\right ] \)
4
\( \left [0, \dfrac {\pi}{4}\right ] \)