There are 4 release of bootstrap version from v1 through v4. See the following chart :
About Bootstrap 4:
How to include or use Bootstrap 4 files on the web page:
There are 2 ways to include Bootstrap files in the website or web application as shown in the following :
Bootstrap 4 from CDN Link:
If you don’t want to download and host Bootstrap 4 files yourself, you can add it from a CDN (Content Delivery Network) Link.
Why use jQuery and popper.js files?
Download Bootstrap 4:
If you want to download and host Bootstrap 4 files yourself, then go to https://getbootstrap.com and follow the instructions there.
Creating Your First Webpage with Bootstrap 4:
In this example, you’ll have made a simple bootstrap 4 webpage that displays “My First webpage” message and 4 columns that have the same size in your web browser.