How to integrate Bootstrap 4 in Ionic 2 or 3?

No app.module.ts

import { BsDropdownModule } from 'ngx-bootstrap';
...
 imports: [
    BrowserModule,
    HttpModule,
    BsDropdownModule.forRoot(MyApp)
...
]

I tried using this library but gave the following error:

  

Sass Error
  It's not clear which file to import for '@import' utils / scss / variables "'. Candidates: utils / scss / variables.scss utils / scss / variables.css
Please delete or rename all but one of these files.
  node_modules / ngx-bootstrap / datepicker / bs-datepicker.scss
  @import 'utils / scss / variables';
  @import 'utils / scss / mixins';