Tenemos la indeterminación de tipo
0/0,
tal que el límite para el numerador es
$$\lim_{x \to 0^+}\left(- \cos{\left(x \right)} + \cos{\left(\sin{\left(x \right)} \right)}\right) = 0$$
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.
$$\lim_{x \to 0^+}\left(\frac{- \cos{\left(x \right)} + \cos{\left(\sin{\left(x \right)} \right)}}{x^{4}}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{\frac{d}{d x} \left(- \cos{\left(x \right)} + \cos{\left(\sin{\left(x \right)} \right)}\right)}{\frac{d}{d x} x^{4}}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{\sin{\left(x \right)} - \sin{\left(\sin{\left(x \right)} \right)} \cos{\left(x \right)}}{4 x^{3}}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{\frac{d}{d x} \left(\sin{\left(x \right)} - \sin{\left(\sin{\left(x \right)} \right)} \cos{\left(x \right)}\right)}{\frac{d}{d x} 4 x^{3}}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{\sin{\left(x \right)} \sin{\left(\sin{\left(x \right)} \right)} - \cos^{2}{\left(x \right)} \cos{\left(\sin{\left(x \right)} \right)} + \cos{\left(x \right)}}{12 x^{2}}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{\frac{d}{d x} \left(\sin{\left(x \right)} \sin{\left(\sin{\left(x \right)} \right)} - \cos^{2}{\left(x \right)} \cos{\left(\sin{\left(x \right)} \right)} + \cos{\left(x \right)}\right)}{\frac{d}{d x} 12 x^{2}}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{3 \sin{\left(x \right)} \cos{\left(x \right)} \cos{\left(\sin{\left(x \right)} \right)} - \sin{\left(x \right)} + \sin{\left(\sin{\left(x \right)} \right)} \cos^{3}{\left(x \right)} + \sin{\left(\sin{\left(x \right)} \right)} \cos{\left(x \right)}}{24 x}\right)$$
=
$$\lim_{x \to 0^+}\left(\frac{\frac{d}{d x} \left(3 \sin{\left(x \right)} \cos{\left(x \right)} \cos{\left(\sin{\left(x \right)} \right)} - \sin{\left(x \right)} + \sin{\left(\sin{\left(x \right)} \right)} \cos^{3}{\left(x \right)} + \sin{\left(\sin{\left(x \right)} \right)} \cos{\left(x \right)}\right)}{\frac{d}{d x} 24 x}\right)$$
=
$$\lim_{x \to 0^+}\left(- \frac{\sin^{2}{\left(x \right)} \cos{\left(\sin{\left(x \right)} \right)}}{8} - \frac{\sin{\left(x \right)} \sin{\left(\sin{\left(x \right)} \right)} \cos^{2}{\left(x \right)}}{4} - \frac{\sin{\left(x \right)} \sin{\left(\sin{\left(x \right)} \right)}}{24} + \frac{\cos^{4}{\left(x \right)} \cos{\left(\sin{\left(x \right)} \right)}}{24} + \frac{\cos^{2}{\left(x \right)} \cos{\left(\sin{\left(x \right)} \right)}}{6} - \frac{\cos{\left(x \right)}}{24}\right)$$
=
$$\lim_{x \to 0^+}\left(- \frac{\sin^{2}{\left(x \right)} \cos{\left(\sin{\left(x \right)} \right)}}{8} - \frac{\sin{\left(x \right)} \sin{\left(\sin{\left(x \right)} \right)} \cos^{2}{\left(x \right)}}{4} - \frac{\sin{\left(x \right)} \sin{\left(\sin{\left(x \right)} \right)}}{24} + \frac{\cos^{4}{\left(x \right)} \cos{\left(\sin{\left(x \right)} \right)}}{24} + \frac{\cos^{2}{\left(x \right)} \cos{\left(\sin{\left(x \right)} \right)}}{6} - \frac{\cos{\left(x \right)}}{24}\right)$$
=
$$\frac{1}{6}$$
Como puedes ver, hemos aplicado el método de l'Hopital (utilizando la derivada del numerador y denominador) 4 vez (veces)