Tomamos como el límite
$$\lim_{x \to \frac{\pi}{4}^+}\left(\frac{x^{4}}{3 x^{3} + \left(- 2 x^{4} - 6\right)}\right)$$
cambiamos
$$\lim_{x \to \frac{\pi}{4}^+}\left(\frac{x^{4}}{3 x^{3} + \left(- 2 x^{4} - 6\right)}\right)$$
=
$$\lim_{x \to \frac{\pi}{4}^+}\left(\frac{x^{4}}{- 2 x^{4} + 3 x^{3} - 6}\right)$$
=
$$\lim_{x \to \frac{\pi}{4}^+}\left(- \frac{x^{4}}{2 x^{4} - 3 x^{3} + 6}\right) = $$
$$- \frac{\left(\frac{\pi}{4}\right)^{4}}{- 3 \left(\frac{\pi}{4}\right)^{3} + 2 \left(\frac{\pi}{4}\right)^{4} + 6} = $$
= -pi^4/(1536 - 12*pi^3 + 2*pi^4)
Entonces la respuesta definitiva es:
$$\lim_{x \to \frac{\pi}{4}^+}\left(\frac{x^{4}}{3 x^{3} + \left(- 2 x^{4} - 6\right)}\right) = - \frac{\pi^{4}}{- 12 \pi^{3} + 2 \pi^{4} + 1536}$$