We need to use the latest css and js files for bootstrap-select. So use version 1.13.0
The old versions of bootstrap-select does not work with bootstrap 4. also do these steps
1. add class="selectpicker" and data-live-search="true"to your select 2. add data-tokens to your options. The live search will look into these data-token element when performing the search.
For more details check this answer on stackoverflow.