Why aren't my bootstrap 4 columns the same height?

Answer 1

You're columns are most likely the same height. It's the projectDescBox that's not. Use the h-100 class (height:100%) to make the inner boxes fill the height.

<div class="projectDescBox h-100">
       <h5>heading</h5>
       <p>a lot longer stuff here. This box should be a lot longer.a lot longer stuff here. This box should be a lot longer.a lot longer stuff here. This box should be a lot longer.</p>
</div>

https://www.codeply.com/go/d8lGpPzUOy

Also, col-xs-* has been replaced with col-* in Bootstrap 4. The multi-tier classes are also unnecessary since there all the same width at each tier.

this: col-8 col-sm-8 col-md-8 col-lg-8 col-xl-8,

is the same as: col-8

You only need to have multi-tier classes when you want different widths at each tier (eg: col-8 col-sm-6 col-lg-4 col-xl-3)