To determine when the equation \(-5 = \frac{4 - 3x}{4 - 3x}\) is undefined, we need to identify when the denominator equals zero. The expression is undefined when:
\[ 4 - 3x = 0 \]
Now, solve for \(x\):
\[ 3x = 4 \ x = \frac{4}{3} \]
So, the equation is undefined when \(x = \frac{4}{3}\).
Thus, the correct answer is:
\(\frac{4}{3}\)