HTML5 introduced semantic elements: these elements provide meaning to both the browsers and search engines like: Google and Bing.

It has an inbuilt support for audio and video files unlike HTML4

It has an inbuilt support for vector graphics like SVG and Canvas unlike HTML4 that can only be supported with the help of Adobe Flash, Silverlight etc.

It supports multiple storage options like Application Cache, SQL Database and Web Storage unlike HTML4 that just provides cookies for application data storage.

It has JS Web Worker API that allows JS and browser interface to run on different threads while HTML4 browser interface and JS run on the same thread which results in performance issues.