Bootstrap 4 has added new css classes for this purpose:
<p class="text-sm-right">Right aligned text on viewports sized SM (small) or wider.</p>
<p class="text-md-right">Right aligned text on viewports sized MD (medium) or wider.</p>
<p class="text-lg-right">Right aligned text on viewports sized LG (large) or wider.</p>
<p class="text-xl-right">Right aligned text on viewports sized XL (extra-large) or wider.</p>