I can see the tinymce4 extention at:
zp-core\zp-extensions\tinymce4

but if I edit a page, I can just see the html-text but no tinymce4 editor at all. Where can I activate the editor? A went through all the plugins in the backend and could see no tinymce4 neither.
Anyone has an idea?

If the plugin is in place it will show up on the plugins tab on the backend. It should be enabled by default. Additionally you might need to set which configuration to use (gallery and Zenpage items) on its plugin options.

If it does not show up something else is wrong like broken upload or something else. Best review your error logs.

sorry for asking the question - you are right - the tinymce4 extension just wasn't activated. After I did it's working. Thanks a lot.