I haven't added it yet to the extnd github repo. Still trying to figure out the best way to do source control with Domino Designer for db specific code such as agents, etc. If you have an older copy of the extnd.nsf db then you can grab it there since I don't think it has changed. Most of the changes for Ext JS 4.x support has been with the JavaScript (which is in github). The only change to agents has been with fixing some bugs with the SearchView agent.
If you don't have an old copy of the nsf then you can get it on OpenNTF.