To simplify the expression \((73^4)^5\), we can use the power of a power property of exponents, which states that \((a^m)^n = a^{m \cdot n}\).
Applying this property to the given expression:
\[ (73^4)^5 = 73^{4 \cdot 5} \]
Now, we calculate the exponent:
\[ 4 \cdot 5 = 20 \]
Thus, we have:
\[ (73^4)^5 = 73^{20} \]
Therefore, the simplified expression is:
\[ \boxed{73^{20}} \]