How do I disable the border on bootstrap 4 cards

simply, you can make the border zero, <div class="card border-0">

If you check the .card class has a border: 1px solid #e5e5e5;

To remove it simply override it to border: none;

There are border utilities that can help with this. From the link:

Subtractive

<span class="border-0"></span>
<span class="border-top-0"></span>
<span class="border-right-0"></span>
<span class="border-bottom-0"></span>
<span class="border-left-0"></span>

So for your scenario, since you want to remove all borders you would add the border-0 CSS Class to your card like so:

<div class="card mb-4 border-0">
</div>