Hi,
I can confirm this. Oxygen Widget for FluentForm is not working after latest 4.0 release by Oxygen Builder.
It does not work for previous widgets or the new ones created after update.
Shortcodes are still working.
I checked page source and what’s happening is that form is not rendering at all.
Here is a screenshot: https://i.imgur.com/Yo4Lu55.png
I can share website link but not here publicly, but I can send it on your email or something.
Looking forward to your response.