Unfortunately 4.1.0 is already available over public Maven and Gradle repos.
Those are alpha versions (and marked as such).
It really took me some effort to migrate from 3 to 4. I don't want to go back to version 3.
You don't need to, stick with the stable 4.0 version.
Could you please provide me with a working client example?
I suggest having a look at Flowdalic/smack-examples · GitHub
Or could you please submit 4.0.5 to the public Maven repositories?
Smack 4.0 is available on OSS sonatype, so it's found on maven central. Current stable is 4.0.6
How about backwards compatibility?
Smack is only API compatible between patch level releases. Most, but I can't guarantee all, API changes on documented in the READMEs. You are able to find out what changed, if it's not documented, by looking the source code history using git.