Why doesn't this structure represent a substituted pentane? In the following ASCI structure (single bond under "$\ce{CH}$" containing $\ce{CH2,CH2, CH3}$)

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

The correct way to name it is "4-methlyheptane".

But when I do it the way I have been taught I get the following "4-propylpentane".
Here are my systematic steps:

Firstly, I count the longest carbon chain. In this case its 5 carbons together so suffix -pentane. Then I look to see if there are any branched chains and number it from left to right. In this case the branched chain is on number 4. The branched chain contains $\ce{CH2-CH2-CH3}$, resp. $\ce{C3H7}$, aka propyl.

Ergo, 4-propylpentane, however, the correct answer as mentioned above is 4-methylheptane.

How is it 'heptane' ? The longest chain has FIVE carbons. I got the '4-' part correct, but how is $\ce{CH2-CH2-CH3}$ Methyl? Is this a mistake in my book?
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.