I don't have access to the textbook you mention, so I resorted to my own sources namely, internet :-)
The Richter magnitude scale (often shortened to Richter scale) was developed to assign a single number to quantify the energy released during an earthquake.
The scale is a base-10 logarithmic scale. The magnitude is defined as the logarithm of the ratio of the amplitude of waves measured by a seismograph to an arbitrary small amplitude.
magnitude 8 = magnitude 4 * 10^(8-4)
= magnitude 4 * 10000
So the statement seems to be erroneous. It should be "The earthquake with a magnitude 8 releases ten thousand times more energy than an earthquake with magnitude 4.”
The original formula is:
M_L = log_10[A] - log_10[A_0(δ)] = log_10[A/A_0(δ)] ... _ means sub, log_10 means
... log base 10
where A is the maximum excursion of the Wood-Anderson seismograph, the empirical function A_0 (A sub 0) depends only on the epicentral distance of the station, δ. In practice, readings from all observing stations are averaged after adjustment with station-specific corrections to obtain the M_L (M sub L) value.
The Mercalli intensity scale is a seismic scale used for measuring the intensity of an earthquake. It measures the effects of an earthquake, and is distinct from the moment magnitude M_w usually reported for an earthquake (sometimes misreported as the Richter magnitude), which is a measure of the energy released. The intensity of an earthquake is not totally determined by its magnitude.
This scale quantifies the effects of an earthquake on the Earth's surface, humans, objects of nature, and man-made structures on a scale from I (not felt) to XII (total destruction). Values depend upon the distance to the earthquake, with the highest intensities being around the epicentral area.
So it seems Mercalli intensity scale is more meaningful in the use of reporting earthquake destruction.
There are several scales which have historically been described as the "Richter scale," especially the local magnitude M_L and the surface wave M_s scale. In addition, the body wave magnitude, m_b, and the moment magnitude, M_w, abbreviated MMS, have been widely used for decades, and a couple of new techniques to measure magnitude are in the development stage.