Use includePaths to import bootstrap into your project

gulpfile.js

var gulp = require('gulp');
var sass = require('gulp-sass');

gulp.task('style', function() {
  return gulp.src('style.scss')
    .pipe(sass({
      includePaths: ["node_modules/bootstrap/scss"]
    }))
    .pipe(gulp.dest('./'));
});

package.json

{
  "name": "new-sass-spike",
  "version": "0.0.1",
  "description": "Just showing how to use the scss files from new bootstrap",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1",
    "build": "gulp style"
  },
  "keywords": [],
  "author": "BigAB <bigab@live.ca> (bigab.net)",
  "license": "MIT",
  "dependencies": {
    "bootstrap": "^4.0.0-alpha.2",
    "gulp": "^3.9.1",
    "gulp-sass": "^2.2.0"
  }
}

style.scss

@import "bootstrap";

body {
  box-sizing: border-box;
}