In RichFaces 4 all request settings related to queue was moved out of components attributes and now customization available using new a4j:attachQueue tag. Next sample shows how requestGroupingId (former similarityGroupingId) could be set for different components making the requests from them similar. Besides setting requestGroupingId - attach queue tag at second button redefines default queue request delay from 500ms to 1500ms.

If you will click the buttons fast - requests will be combined and only last click will be processed