To simplify the expression \((73^4)^5\) using the Power Rule of Exponents, we apply the rule which states that \((a^m)^n = a^{m \cdot n}\).
In this case, we have:
\[ (73^4)^5 = 73^{4 \cdot 5} = 73^{20} \]
Thus, the simplified expression is:
\[ 73^{20} \]