It's not clear what we can use for free on this new 4.0 version. I run the docker image and the Chat and Comments were there, but I'm not sure one can use it for free. Is the coauthoring still free like in the 3.x version?
Yes, the features you describe are totally free.
The difference in document editing between the Enterprise and Free Edition are Mobile Apps which are not included in Free Edition.
Also the difference is in the connections limit - 20 opened documents (sessions) in an Open Source version and 1 conversion thread,
The comparison of ONLYOFFICE Editions is available here
We would like to note that ONLYOFFICE Community Edition is released under an AGPL license, which must be also applied for your product in case of integration or for your code in case of adding some parts.