AFAIK, there are no new themes published in the App Store or elsewhere that are designed for version 4. The only v4 themes available are those supplied with the Modeler and automatically expanded to your deployment/web folder when you run a project.

The easiest way to get these is to run a project from the Modeler with no custom themes specified, then copy the files from the deployment/web and deployment/web/ui folders to a separate location so they do not get overwritten to have a look at what they contain.

The source of these files is in deployment.mxz in your modeler installation directory.

Edit: It looks like there is a new v4 theme available in the App Store now at workshop theme