The expression \(-5 = \frac{4 - 3x}{4 - 3x}\) is undefined when the denominator is equal to zero. To find the value that makes the denominator \(4 - 3x\) zero, we can set it equal to zero and solve for \(x\):
\[ 4 - 3x = 0 \]
Now, solve for \(x\):
\[ 3x = 4 \] \[ x = \frac{4}{3} \]
Therefore, the value that makes the equation undefined is \(x = \frac{4}{3}\).