Difference Between Bootstrap 3 and 4

Bootstrap 3 Source CSS Files is LESS.
Bootstrap 4 Source CSS files Converted in SCSS.

Bootstrap 3 has 4 grid system (col-xs-3, col-sm-3, col-md-3, col-lg-3).
Bootstrap 4 has 5 grid system (.col-, .col-sm-3, .col-md-3, .col-lg-3, .col-xl-3). Bootstrap 4 has removed the xs from the lowest break point. Therefore, (col-) covers all devices.

Bootstrap 3 Offset class is like this (col-md-offset-4).
Bootstrap 4 has changed the offset class (offset-md-4).

In Bootstrap 3 we are used this (.img-responsive) class for responsive image.
Bootstrap 4 has changes the class for responsive image (.img-fluid).

In Bootstrap 3 we are used (.input-lg) and (.input-sm) to increase and decrease the input size.
Bootstrap 4 has changed to (.form-control-lg) and (.form-control-sm) to increase and decrease the input size.