Notam 1/x^4=t
[tex] \displaystyle \lim_{x \to \infty} \frac{e^{\frac{1}{x^2+1}}-e^{\frac{1}{x^2}}}{\frac{1}{x^4}}=\\
= \lim_{x \to 0}\frac{e^{\frac{\sqrt{t}}{1+\sqrt{t}}}-e^{\sqrt{t}}}{t}=\\
= \lim_{x \to 0}\frac{e^{\sqrt{t}}(e^{\frac{-t}{1+\sqrt{t}}}-1)}{t}=-1[/tex]