Why doesn't this structure represent a substituted pentane?
You need to find the longest continuous carbon chain of:

$$
\begin{align*}
\ce{CH3-CH2-CH2-&amp;CH-CH3}\\
&amp;|\\
\ce{&amp;CH2-CH2-CH3}
\end{align*}
$$

Stretch that out to $\ce{CH3CH2CH2CH(CH3)CH2CH2CH3}$ (with the $\ce{-CH3}$ coming off of the $\ce{CH}$) and you'll see that the book is correct.  The "side chain" of $\ce{-CH2CH2CH3}$ is actually part of the main carbon chain.  As far as the numbering, counting from either side give the 4- position due to the symmetry of the molecule.