Bootstrap 4 is a widely used front-end framework for creating responsive websites and applications, featuring built-in HTML, CSS components, and optional JavaScript extensions. Key features include an improved grid system, new typography options, and cross-browser compatibility, making it suitable for both beginners and advanced users. The document outlines five real-world projects designed to enhance learning, including a photo sharing site, a professional resume, a social networking platform, an agency site, and a lightbox project, all offered at a promotional price until November 26, 2017.