The estimated value is a significant digit in this measured quantity for the wood's length. Thus, the measurement has four significant digits. The last digit given for any measurement is the uncertain or estimated digit. It is uncertain because it is estimated. The last digit given for a measurement is always assumed to be estimated and it is significant.
Zeroes are only sometimes significant when reported in a measurement.
Not all zeroes are significant. For example, if you reported the measured length of an object to be 0.0030400 m, it would have only five significant digits. The first three zeroes only serve to locate the decimal point and are not significant. The other three zeroes are significant. The last zero is the estimated digit and it is significant.