To simplify \((5^4)^3\) to a single power of 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:
\[ (5^4)^3 = 5^{4 \cdot 3} = 5^{12} \]
Therefore, \((5^4)^3\) simplifies to \(5^{12}\).