idoetz Hi Ralf
You can check for Joomla 4 status of our templates at this page https://www.joomlart.com/joomla4-status/ .
We are currently working on the new update for JA Mono and hope to release it within this month.
Regarding the topic https://www.joomlart.com/forums/d/35403-showing-video-in-masthead-position there wasn't a definite answer in that thread. Our tech team will check further and get back to you with a confirmation soon.
UPDATE: It is possible to show video in masthead section of JA Mono template. If you try it and have problem please ask for further assistance in the forum here, our team will be glad to provide support.