Tenemos la indeterminación de tipo
0/0,
tal que el límite para el numerador es
$$\lim_{x \to 0^+}\left(x^{4} \sin^{2}{\left(x \right)} - \tan^{2}{\left(x \right)}\right) = 0$$
y el límite para el denominador es
$$\lim_{x \to 0^+} x^{4} = 0$$
Vamos a probar las derivadas del numerador y denominador hasta eliminar la indeterminación.
$$\lim_{x \to 0^+}\left(\sin^{2}{\left(x \right)} - \frac{\tan^{2}{\left(x \right)}}{x^{4}}\right)$$
=
Introducimos una pequeña modificación de la función bajo el signo del límite
$$\lim_{x \to 0^+}\left(\frac{x^{4} \sin^{2}{\left(x \right)} - \tan^{2}{\left(x \right)}}{x^{4}}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{\frac{d}{d x} \left(x^{4} \sin^{2}{\left(x \right)} - \tan^{2}{\left(x \right)}\right)}{\frac{d}{d x} x^{4}}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{2 x^{4} \sin{\left(x \right)} \cos{\left(x \right)} + 4 x^{3} \sin^{2}{\left(x \right)} - 2 \tan^{3}{\left(x \right)} - 2 \tan{\left(x \right)}}{4 x^{3}}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{\frac{d}{d x} \left(2 x^{4} \sin{\left(x \right)} \cos{\left(x \right)} + 4 x^{3} \sin^{2}{\left(x \right)} - 2 \tan^{3}{\left(x \right)} - 2 \tan{\left(x \right)}\right)}{\frac{d}{d x} 4 x^{3}}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{- 2 x^{4} \sin^{2}{\left(x \right)} + 2 x^{4} \cos^{2}{\left(x \right)} + 16 x^{3} \sin{\left(x \right)} \cos{\left(x \right)} + 12 x^{2} \sin^{2}{\left(x \right)} - 6 \tan^{4}{\left(x \right)} - 8 \tan^{2}{\left(x \right)} - 2}{12 x^{2}}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{\frac{d}{d x} \left(- 2 x^{4} \sin^{2}{\left(x \right)} + 2 x^{4} \cos^{2}{\left(x \right)} + 16 x^{3} \sin{\left(x \right)} \cos{\left(x \right)} + 12 x^{2} \sin^{2}{\left(x \right)} - 6 \tan^{4}{\left(x \right)} - 8 \tan^{2}{\left(x \right)} - 2\right)}{\frac{d}{d x} 12 x^{2}}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{- 8 x^{4} \sin{\left(x \right)} \cos{\left(x \right)} - 24 x^{3} \sin^{2}{\left(x \right)} + 24 x^{3} \cos^{2}{\left(x \right)} + 72 x^{2} \sin{\left(x \right)} \cos{\left(x \right)} + 24 x \sin^{2}{\left(x \right)} - 24 \tan^{5}{\left(x \right)} - 40 \tan^{3}{\left(x \right)} - 16 \tan{\left(x \right)}}{24 x}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{\frac{d}{d x} \left(- 8 x^{4} \sin{\left(x \right)} \cos{\left(x \right)} - 24 x^{3} \sin^{2}{\left(x \right)} + 24 x^{3} \cos^{2}{\left(x \right)} + 72 x^{2} \sin{\left(x \right)} \cos{\left(x \right)} + 24 x \sin^{2}{\left(x \right)} - 24 \tan^{5}{\left(x \right)} - 40 \tan^{3}{\left(x \right)} - 16 \tan{\left(x \right)}\right)}{\frac{d}{d x} 24 x}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{x^{4} \sin^{2}{\left(x \right)}}{3} - \frac{x^{4} \cos^{2}{\left(x \right)}}{3} - \frac{16 x^{3} \sin{\left(x \right)} \cos{\left(x \right)}}{3} - 6 x^{2} \sin^{2}{\left(x \right)} + 6 x^{2} \cos^{2}{\left(x \right)} + 8 x \sin{\left(x \right)} \cos{\left(x \right)} + \sin^{2}{\left(x \right)} - 5 \tan^{6}{\left(x \right)} - 10 \tan^{4}{\left(x \right)} - \frac{17 \tan^{2}{\left(x \right)}}{3} - \frac{2}{3}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{x^{4} \sin^{2}{\left(x \right)}}{3} - \frac{x^{4} \cos^{2}{\left(x \right)}}{3} - \frac{16 x^{3} \sin{\left(x \right)} \cos{\left(x \right)}}{3} - 6 x^{2} \sin^{2}{\left(x \right)} + 6 x^{2} \cos^{2}{\left(x \right)} + 8 x \sin{\left(x \right)} \cos{\left(x \right)} + \sin^{2}{\left(x \right)} - 5 \tan^{6}{\left(x \right)} - 10 \tan^{4}{\left(x \right)} - \frac{17 \tan^{2}{\left(x \right)}}{3} - \frac{2}{3}\right)$$
=
$$-\infty$$
Como puedes ver, hemos aplicado el método de l'Hopital (utilizando la derivada del numerador y denominador) 4 vez (veces)