Columns have padding to create the gutter (spacing between columns). Therefore, the background colors will blends together unless you put the background on the contents of the columns. Also, use col-md-6
not col-md-5
...
<section class="row">
<div class="col-12 mb-3">
<div class="bg-info" style="height: 175px;"></div>
</div>
<!-- one -->
<div class="col-12 col-md-6 mb-3">
<div class="bg-info" style="height: 100px;"></div>
</div>
<!-- two -->
<div class="col-12 col-md-6 mb-md-4 mb-3">
<div class="bg-info" style="height: 100px;"></div>
</div>
<!-- three -->
<div class="col-12 col-md-6 mb-3">
<div class="bg-info" style="height: 100px;"></div>
</div>
<!-- four -->
<div class="col-12 col-md-6 mb-3">
<div class="bg-info" style="height: 100px;"></div>
</div>
</section>
https://www.codeply.com/p/QTN0GCfLbc