On the Bootstrap 3 media queries documentation it says:
We use the following media queries in our Less files to create the key breakpoints in our grid
Here are the selectors used in Bootstrap 4. There is no "lowest" setting in BS4 because "extra small" is the default. I.e. you would first code the XS size and then have these media overrides afterwards.
@media(min-width:576px){}
@media(min-width:768px){}
@media(min-width:992px){}
@media(min-width:1200px){}