Cite or reformulate perfectly formulated sentences? I'm just writing my bachelors thesis and I need to reference to a book for an in-depth derivation of something.
The concluding sentence in this work is just perfectly formulated, there is no way of making it shorter or more precise. I wonder whether I should write something like

As [4] points out, it can be shown that
the enhancement in the second-harmonic power is given [...]

With the bold text being the quote from the book or reformulate the sentence, making it somewhat less precise? Is this even enough attribution when copying the sentence verbatim or do I need to put quotation marks?
Yes, you should use quotation marks for direct quotes unless you use some other clearly expressed convention for quotes. Bolding doesn't naturally imply quoting. With quotes you may still want to bold the text, or not.
Also, your &quot;citation&quot; is a bit weak if it only points to the book and not to a particular place within the book.