Distinct Horizontal Asymptotes

$f(x) = \frac{x^3 - x}{x^2 + 1}$

$f(x) = \frac{x}{\sqrt{x^2 + 1}}$

$f(x) = \frac{2x^2 + 3}{x^2 - 4}$

$f(x) = \frac{x^4 + 2x^2}{x^4 - 1}$