Bootstrap 4 released in August 2017 and added new features of flex modal for grid system and even added new components.

Let us see them one by one based on new features, improved and deprecated:

New Features

• Bootstrap 4 came with .col-form-label-sm and .col-form-label-lg to increase or decrease the size of a label and match the size of the relevant form control.
• For offset columns, Bootstrap 4 use offset-*-* classes.
• Add dark tables with the .table-dark class in Bootstrap 4
• For increasing or decreasing the input control size, Bootstrap 4 has .form-control-lg and .form-control-sm classes.
• Create table head styles with .thead-light and .thead-dark classes.
• Bootstrap 4 INTRODUCED the .btn-outline-* classes for styling buttons with an outline color.
• To disable Menu Items in Bootstrap 4, you need to use the .DISABLED class . Set the class to the <a> to disable.
• To fix navbars to the top and bottom, Bootstrap 4 introduced .fixed-top and .fixed-bottom classes.
• Bootstrap 4 came with .jumbotron-fluid class for full-width jumbotrons.
• For creating carousel item with Bootstrap 4, use the .carousel-item class.

Enhanced

• Bootstrap 4 UPDATED GLOBAL Form Size of 16px.
• To create responsive images with Bootstrap 4, use .img-fluid class. The .img-responsive class was used in Bootstrap 3 for creating responsive images.
• Introduction of cards in Bootstrap 4 replaced the functionality provided by Panels.
• For checkbox and radio button, Bootstrap 4 use .form-check, .form-check-label, .form-check-input, and .form-check-inline. Bootstrap 3 used .radio, .radio-inline, .checkbox, or .checkbox-inline classes.
• Bootstrap 4 introduced Cards to replace the functionality provided by WELLS in Bootstrap 3.

Deprecated (Not Supported)

• Bootstrap 4 do not support any class for extra small button groups.
• The .page-header class isn’t supported in Bootstrap 4.
• The .btn-group-justified class is not supported in Bootstrap 4.
• Bootstrap 4 deprecated the .btn-xs class. Rest, .btn-sm and .btn-lg are still available to resize buttons.