Just my thoughts!
Why? Because 5.4 branch is already in GitHub. However, they may be still waiting for some things, like succesfully adding Vision Pro support, promised to be included in 5.4.
Also, a session from the GDC schedule:
And what about Fab? It’s already available in UEFN as a plugin and, that same plugin, is already available too in UE5.4 GitHub version, even if not ‘accesible’, so I think they will be released together.
Can’t wait!!