Does bootstrap 4 have a built in horizontal divider?

Bootstrap 4 define a CSS style for the HTML built-in horizontal divider <hr />, so just use it.

You can also customize margin with spacing utilities: mt for margin top, mb for margin bottom and my for margin top and bottom. The integer represent spacing 1 for small margin and 5 for huge margin. Here is an example:

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.5.2/css/bootstrap.min.css">

<hr class="mt-2 mb-3"/>
<!-- OR -->
<hr class="my-12"/>
<!-- It's like -->
<hr class="mt-3 mb-3"/>

I used to be using just a div with border-top like:

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.5.2/css/bootstrap.min.css">

<div class="border-top my-3"></div>

but it's a silly method to make the work done, and you can have some issues. So just use <hr />.

HTML already has a built-in horizontal divider called <hr/> (short for "horizontal rule"). Bootstrap styles it like this:

hr {
  margin-top: 1rem;
  margin-bottom: 1rem;
  border: 0;
  border-top: 1px solid rgba(0, 0, 0, 0.1);
}

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" />
<p>
   Some text
   <hr/>
   More text
</p>