RichFaces 4 is a lightweight, open source framework for JSF 2 that provides rich Ajax components and functionality. It extends JSF 2 to add Ajax, client-side validation, and a component development kit. Some key components include <a4j:ajax> for sending Ajax requests, <a4j:commandButton> and <a4j:commandLink> for buttons and links with Ajax behavior, and <a4j:jsFunction> for firing Ajax requests from JavaScript. RichFaces 4 allows building rich client applications with JSF 2.