The accepted answer doesn’t provide a solution to the posters question.
Bootstrap 4 .rows are flex containers and the .col’s in the row are the flex items. There are a bunch of utility classes that they provide to control the flex containers and items.
To prevent the columns in your row from stacking add the .flex-nowrap class to your row:
<form class="row flex-nowrap">
...
</form>
Reference: Bootstrap 4 – Flex Doc’s