Bootstrap on wordpress is a blank bootstrap 3.3.7 ready wordpress theme.
For full documentation go to the Bootstrap on Wordpress website.
You can find bootstrap 4 on WordPress here.
Download the files, with the "Download ZIP" button.
You will have to unzip it, zip the folder witch contains the files, and the theme is ready to install.
You can also install the files true ftp, you will have to drop the folder witch contains the files in the themes directory(wp-content/themes).
all you have to do is activate the theme true the admin panel(display/themes).