I got a little lost along the way…on my V4 I still have OS 2109 and neither Pianoteq nor Mod-UI. To update the operating system can I do it from Webconf or do I have to download it to the SD and reinstall it from scratch?
Download and reinstall. Your version is too old for updating.
I am going to buy a Zynthian V4
As first, do I have to update or it is better load the latest operating system?