To simplify the expression \((73^4)^5\) using the Power Rule of Exponents, we apply the rule that states \((a^m)^n = a^{m \cdot n}\).
Here, \(a = 73\), \(m = 4\), and \(n = 5\).
So, we multiply the exponents:
\[ (73^4)^5 = 73^{4 \cdot 5} = 73^{20}. \]
Thus, the simplified expression is:
\[ \boxed{73^{20}}. \]