To calculate the percent error of a measurement, you need to know both the measured value and the true value (or accepted value) of what you are measuring. The formula for percent error is:
\[ \text{Percent Error} = \left(\frac{|\text{Measured Value} - \text{True Value}|}{\text{True Value}}\right) \times 100% \]
Since you have provided only the measurement of 4 cm without a true value, I cannot calculate the percent error.
If you can provide the true value, I can help you with the calculation!