Aside from what I've gone over already, there are a number of other changes you need to be aware of when migrating to Bootstrap 4. Let's go through them all below.
The .page-header class has been dropped from version 4. Instead, you should look at using the new display CSS classes on your headers if you want to give them a heading look and feel.
If you've ever used responsive images in the past, the class name has changed. Previously, the class name was .image-responsive but it is now named .image-fluid. You'll need to update that class anywhere it is used.
For the table component, a few class names have changed and there are some new classes you can use.
If you would like to create a responsive table, you can now simply add the class .table-responsive to the <table> tag. Previously, you had to wrap the class around the <table> tag. If migrating, you'll need to update your HTML markup to the new format...