Bootstrap 4の主な変更点とファーストインプレッション ref: http://qiita.com/youthkee/items/e9e105e35526f908f48a

_mixins.scss

// Toggles
//
// Used in conjunction with global variables to enable certain theme features.

@mixin border-radius($radius: $border-radius) {
  @if $enable-rounded {
    border-radius: $radius;
  }
}

@mixin box-shadow($shadow...) {
  @if $enable-shadows {
    box-shadow: $shadow;
  }
}

@mixin transition($transition...) {
  @if $enable-transitions {
    transition: $transition;
  }
}

_utilities-spacing.scss

.m-t { margin-top:    $spacer-y !important; }
.m-t-md { margin-top:    ($spacer-y * 1.5) !important; }
.m-t-lg { margin-top:    ($spacer-y * 3) !important; }

_variables.scss

$spacer:                     1rem !default;
$spacer-x:                   $spacer !default;
$spacer-y:                   $spacer !default;
$border-width:               .0625rem !default;

_variables.scss（変更前）

$enable-flex:               false !default;

_variables.scss（変更後）

$enable-flex:               true !default;

bootstrap.css

html {
  font-size: 16px;
  -ms-overflow-style: scrollbar;
  -webkit-tap-highlight-color: transparent;
}

file0.txt

$ bower install bootstrap#4.0.0-alpha

Gruntfile.js（Libsass使用時）

  // CSS distribution task.
  // Supported Compilers: sass (Ruby) and libsass.
  (function (sassCompilerName) {
    require('./grunt/bs-sass-compile/' + sassCompilerName + '.js')(grunt);
  })(process.env.TWBS_SASS || 'libsass');

Gruntfile.js（RubySass使用時）

  // CSS distribution task.
  // Supported Compilers: sass (Ruby) and libsass.
  (function (sassCompilerName) {
    require('./grunt/bs-sass-compile/' + sassCompilerName + '.js')(grunt);
  })(process.env.TWBS_SASS || 'sass');