Evaluate \( \log _{3} 81 \) \( \log _{3} 81=\square \)

To evaluate \( \log_{3} 81 \), we first express 81 as a power of 3. We know that \( 81 = 3^4 \). Therefore, we can rewrite the logarithm: \[ \log_{3} 81 = \log_{3}(3^4). \] Using the logarithmic identity \( \log_{b}(b^k) = k \), we have: \[ \log_{3}(3^4) = 4. \] Thus, \[ \log_{3} 81 = 4. \] So, \( \log_{3} 81 = \boxed{4} \).