Differences Between Bootstrap 3 & 4

Component	Bootstrap 3	Bootstrap 4
Global
Source CSS Files	LESS	SCSS
Primary CSS Unit	px	rem
Media Queries Unit	px	px
Global Font Size	14px	16px
Default Fonts	Helvetica Neue, Helvetica, Arial, sans-serif	Uses a "native font stack" (user's system fonts), with a fallback to Helvetica Neue, Arial, and sans-serif
Grids
Grid Tiers	4 tier grid system (xs, sm, md, lg)	5 tier grid system (xs, sm, md, lg, xl). Bootstrap 4 has removed the xs from the lowest break point. Therefore, col-* covers all devices (no need to specify the size in this case).
Offsetting Columns	Uses col-*-offset-* classes to offset columns. For example, col-md-offset-4.	Uses offset-*-* classes to offset columns. For example, offset-md-4.
Tables
Dark/inverse Tables	Not supported.	Added dark/inverse tables with the .table-dark class. Note: Prior to the Beta 2 release, these required the .table-inverse class. This class has been replaced with .table-dark in Beta 2.
Table Head Styles	Not supported.	Added table head styles with the .thead-light and .thead-dark classes. Note: Prior to the Beta 2 release, these required the .table-default class and the .table-dark class. These classes have been replaced with .table-light and .table-dark in Beta 2.
Condensed Tables	.table-condensed	.table-sm
Contextual Classes	Bootstrap 3 doesn't use the .table- prefix for its contextual classes. For example, Bootstrap 3 uses .active whereas Bootstrap 4 uses .table-active. Other than that, both versions use the same 5 contextual keywords (active, success, info, warning, danger).	Added the .table- prefix for its contextual classes.
Responsive Tables	The .table-responsive class must be added to a parent div element.	Can add .table-responsive to the actual table element. Beta 2 has also introduced the .table-responsive-* classes that can be used to specify a particular breakpoint. These are .table-responsive-sm, .table-responsive-md, .table-responsive-lg, and .table-responsive-xl.
Reflow Tables	Not supported.	Added reflow tables with the .table-reflow class. Reflow tables have now been dropped.
Forms
Horizontal Forms	Horizontal forms require the .form-horizontal class. Forms don't require .row when using grids (although this class is still a requirement on Bootstrap 3 grids in general).	Bootstrap 4 dropped the .form-horizontal class — it is no longer needed to display horizontal forms. Forms require the .row class when using grids.
	Use .control-label when using grids for form layout.	Bootstrap 4 uses .col-form-label * when using grids for form layout. * Note that Bootstrap 4 initially used .form-control-label but subesquently changed this to .col-form-label.
Checkboxes and Radio Buttons	Uses .radio, .radio-inline, .checkbox, or .checkbox-inline to display checkboxes and radio buttons.	Uses .form-check, .form-check-label, .form-check-input, and .form-check-inline.
Form Control Size	Use .input-lg and .input-sm to increase or decrease the size of an input control.	Use .form-control-lg and .form-control-sm to increase or decrease the size of an input control.
Help Text	Use the .help-block class to display help text.	Bootstrap 4 uses the .form-text class to display help text. Bootstrap 4 forms don't have an official class for help text. Instead, the .text-muted class was introduced for more flexibility.
Validation and Feedback Icons	Includes validation styles for error, warning, and success states on form controls (for example, .has-warning). To present icons on the input fields using Bootstrap 3, use glyphicons.	Validation styles are not available for Bootstrap 4 forms. Use custom Bootstrap form validation messages instead.
Legends	No classes for styling the form legends.	Provides the option of using .col-form-legend on legend elements to style it more like a label.
Custom Forms	Not supported.	Bootstrap 4 introduced custom forms — completely custom form elements that replace the browser defaults.
Buttons
Styles	Includes the .btn-default and .btn-info classes. The .btn-secondary class isn't available in Bootstrap 3.	Introduced the .btn-secondary, .btn-light, and .btn-dark classes. Dropped the .btn-default class. Note that the .btn-info class was initially dropped in Bootstrap 4 but then reappeared again.
Outline Buttons	Not supported.	Introduced the .btn-outline-* classes for styling buttons with an outline color. Introduced the .btn-*-outline classes for styling buttons with an outline color. (The .btn-*-outline classes became .btn-outline-* in Alpha 3).
Button Sizes	The .btn-xs class is available.	Dropped the .btn-xs class (only .btn-sm and .btn-lg are available now).
Images
Responsive Images	Use .img-responsive class.	Use .img-fluid class.
Image Alignment	Use .pull-right, .pull-left, and .center-block helper classes.	Uses .m-x-auto instead of .center-block to align block-level images. Can also use the various .pull-*-right and .pull-*-left responsive helper classes, as well as the .text-*-left, .text-*-center, and .text-*-right helper classes on the image's parent. Can use the various .pull-*-none classes to disable floating.
Media Objects
Classes	Includes many different classes for media objects, including .media, .media-body .media-object, .media-heading, .media-right, .media-left, and .media-list and .media-body.	Uses just .media class. Margins can be applied using spacer utilities. Media objects are flexbox enabled in Bootstrap 4, so the various flexbox classes can also be applied (such as reordering, etc).
Dropdowns
Structure	Apply dropdowns to lists (i.e. using ul and li).	Dropdowns can be built with ul or divs. Apply the .dropdown-item to a a or button element and wrap them all in a div (or ul) with a .dropdown-menu class applied.
Menu Headers	Apply .dropdown-header to the li tag.	Apply .dropdown-header to h1 - h2 tags (as Bootstrap no longer uses li tags to build dropdowns).
Dividers	Apply the .divider class to the li element (because it used lists to build dropdowns).	Apply the .dropdown-divider to the div element.
Disabled Menu Items	Apply the .disabled class to the li element.	Apply the .disabled class to the a element.
Button Groups
Justified?	Supports justified button groups (via the .btn-group-justified class).	Not supported.
Extra Small?	Supports extra small button groups (via the .btn-group-xs class).	Not supported (dropped the .btn-group-xs class).
Navs
Inline Navs	There is no .nav-inline class.	Can use the .nav-inline class to explicitly specify navs to be displayed inline.
Navbars
Colors	Limited (preset) color options. Supports inverse navbars but not the other classes.	New (preset) color options. Introduced the .navbar-light and the .navbar-dark classes, and allows the .bg-* classes to be used on navbars.
Navbar Alignment	Use .navbar-right, .navbar-left to align components within the navbar.	Use the various .pull-*-right and .pull-*-left responsive helper classes. Can also use the various .pull-*-none classes to disable floating. Can either use spacing utilities such as .mr-auto, or any of the flexbox alignment utilities.
Navbar Forms	Add the .navbar-form class to forms within navbars.	Bootstrap 4 dropped the .navbar-form class. It's no longer necessary.
Fixed Navbars	Uses .navbar-fixed-top and .navbar-fixed-bottom to fix navbars to the top or bottom.	Uses .fixed-top and .fixed-bottom.
Pagination
Default Pagination	Only requires .pagination to be added (to the ul element that represents the list of pages).	Must also add .page-item to each li element and .page-link to each a element.
Pagers	Uses .previous and .next for aligning pagers.	Pagers have been dropped in Bootstrap 4 (Alpha 3). Uses .pager-prev and .pager-next for aligning pagers.
Labels
Pill Labels	The .label-pill class is not available. However, Bootstrap 3 does have badges (which achieves a similar visual effect).	Labels have been replaced by badges in Bootstrap 4. Badges can use the .badge-pill for rounded corners.
Tags
Supported?	No. "Tags" are called "Labels" in Bootstrap 3 (i.e. they use the .label class).	Yes. The .tag class was introduced in Bootstrap 4 (Alpha 3) to replace the .label class. This was done to disambiguate from the label element. Tags have now been renamed to "badges". These replace labels from Bootstrap 3.
Jumbotron
Full-Width	The .jumbotron-fluid class is not required on full-width jumbotrons.	Introduced the .jumbotron-fluid class for full-width jumbotrons.
Progress Bars
Uses progress?	Doesn't use the progress for progress bars. Instead, applies progress bar classes to nested div elements.	Uses the HTML5 progress element when working with progress bars. Using the progress element was abandoned in Alpha 6. Bootstrap 4 now uses the div element again.
Glyphicons
Supported?	Supported.	Not supported.
Typography
Blockquotes	Bootstrap styles are applied to the blockquote element by default.	Introduced the .blockquote class for styling the blockquote element (i.e. styling this element is now opt-in).
Blockquote Alignment	Use .blockquote-reverse to align a blockquote to the right.	Use the text utilities to align blockquotes (.text-center and .text-right).
Page Headers	The .page-header class is supported.	The .page-header class is not supported.
Description Lists	The .dl-horizontal class was used to declare a horizontal list.	Horizontal lists are now declared with the .row class on the dl tag, then any of the grid system's predefined classes to the dt and dd tags.
Non-Responsive Usage
Supported?	Supported. You can specify a layout to be non-responsive.	Not supported.
List Groups
Linked List Items / Button List Items	Apply .list-group-item to the a element.	Apply .list-group-item-action to the a element.
Collapse
Show content	Uses .in to have content expanded upon page load.	Uses .show to have content expanded upon page load.
Cards
Supported?	Not supported.	Introduced in Bootstrap 4. Cards replace functionality that was previously provided by panels, wells, and thumbnails.
Panels
Supported?	Supported.	Not supported. Use cards instead.
Wells
Supported?	Supported.	Not supported. Use cards instead.
Thumbnails
Supported?	Supported.	Not supported. Use cards instead.
Breadcrumbs
Classes	Uses the .breadcrumb class against the ul tag.	Also requires .breadcrumb-item to be applied against all li elements that make up the breadcrumb. Breadcrumbs can also be used outside of lists. For example, the .breadcrumb-item could be applied to a bunch of a elements that are wrapped in a .breadcrumb element.
Carousels
Carousel Item	Use .item class.	Use .carousel-item class.
Affix
Supported?	Yes.	No.