Direct WP to a custom “single.php” file?

I have created a custom “single.php” file that I would like to use to display a certain category of posts. (It’s named “category_single.php”)

These posts appear on a page generated by a custom category template (“category-4.php”), which is called via a wp_list_categories link in “header.php”.

It’s not clear to me exactly where WP calls for the “single.php” file to begin with, and because of that I can’t figure out where to start!

I’ve read a ton of the conditional tags material in the WP codex, and it does seem as though that would be a useful method — but I can’t see where I would put the conditional code to call for the custom “category_single.php” file instead of “single.php”.