Bootstrap 4 - change the carousel indicators into dots

Yes, I think you need to write custom CSS for doing lines into dots.
You just need to override two properties (width and height) and add a new one, border-radius, to .carousel-indicators li.
Make width and height values equal eg: 10px each and give a border-radius of 100%.

.carousel-indicators li {
  width: 10px;
  height: 10px;
  border-radius: 100%;
}