Bootstrap 4 Modal Center Content

Below I have added text-center d-block to the header and d-inline-block to the title.

The problem was that the header was flex with justify-content space-between which pushed your title to the left.

<!-- Button trigger modal -->
<button type="button" class="btn btn-primary" data-toggle="modal" data-target="#exampleModal">
  Launch demo modal
</button>

<!-- Modal -->
<div class="modal fade" id="exampleModal" tabindex="-1" role="dialog" aria-labelledby="exampleModalLabel" aria-hidden="true">
  <div class="modal-dialog" role="document">
    <div class="modal-content">
      <div class="modal-header text-center d-block">
        <h5 class="modal-title d-inline-block" id="exampleModalLabel">Modal title</h5>
        <button type="button" class="close" data-dismiss="modal" aria-label="Close">
          <span aria-hidden="true">&times;</span>
        </button>
      </div>
      <div class="modal-body">
        ...
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-secondary" data-dismiss="modal">Close</button>
        <button type="button" class="btn btn-primary">Save changes</button>
      </div>
    </div>
  </div>
</div>

Example Bootply

The modal-header is display:flex so centering its' content (like the modal-title) works differently. You can use mx-auto but then centering is relative to the close button so it's not exactly centered.

One option is to make the header display:block (d-block) and use text-center.

https://jsfiddle.net/44v0b25k/

<div class="modal fade" id="exampleModal" tabindex="-1" role="dialog" aria-labelledby="exampleModalLabel" aria-hidden="true">
  <div class="modal-dialog" role="document">
    <div class="modal-content">
      <div class="modal-header d-block">
        <button type="button" class="close float-right" data-dismiss="modal" aria-label="Close">
          <span aria-hidden="true">&times;</span>
        </button>
        <h5 class="modal-title text-center" id="exampleModalLabel">Modal title</h5>
      </div>
      <div class="modal-body">
        ...
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-secondary" data-dismiss="modal">Close</button>
        <button type="button" class="btn btn-primary">Save changes</button>
      </div>
    </div>
  </div>
</div>

Another option is to use w-100 on the modal-title so that it's full width, and text-center will also work.

<div class="modal-header">
     <h5 class="modal-title w-100 text-center" id="exampleModalLabel">Modal title</h5>
     <button type="button" class="close" data-dismiss="modal" aria-label="Close">
        <span aria-hidden="true">&times;</span>
     </button>
</div>

https://jsfiddle.net/306ob2e5/