In Bootstrap 4, if you want to do something like this:
Mobile | Desktop
-----------------------------
A | A
C | B C
B | D
D |
You need to reverse the order of B then C then apply order-{breakpoint}-first
to B. And apply two different settings, one that will make them share the same cols and other that will make them take the full width of the 12 cols:
Smaller screens: 12 cols to B and 12 cols to C
Larger screens: 12 cols between the sum of them (B + C = 12)
Like this
<div class='row no-gutters'>
<div class='col-12'>
A
</div>
<div class='col-12'>
<div class='row no-gutters'>
<div class='col-12 col-md-6'>
C
</div>
<div class='col-12 col-md-6 order-md-first'>
B
</div>
</div>
</div>
<div class='col-12'>
D
</div>
</div>