If \( U_n=\left(1+\frac{1}{n^2}\right)^1\left(1+\frac{2^2}{n^2}\right)^2\cdots\left(1+\frac{n^2}{n^2}\right)^n \), then \( \displaystyle \lim_{n\to \infty}(U_n)^{-\frac{4}{n^2}} \) is equal to:
\( \frac{4}{e} \)
\( \frac{e^2}{16} \)
\( \frac{16}{e^2} \)
\( \frac{4}{e^2} \)