[split] What's it look like?

See https://forum.codeigniter.com/forum-33.html

Not sure what you mean by "fixed", or by "looking & acting like CI3".
- if you refer to runnable out of the box, you got it
- if you refer to CI3 with namespaces, that's not it
- if you refer to backwards compatibility, nope
- if you are thinking PHP5 compatibility, that's not there either
- if you refer to the same principles that developers love CI3 for, you got it

"HMVC", by some interpretations, is either fully supported or redundant.
There is no specific HMVC implementation, unlike the addin for CI3.
Controllers, models, etc, etc can all be loaded from whatever folder structure tickles your fancy, so yes it "does" HMVC.

Compared to other frameworks, I see many similar abstractions, but the other mentioned frameworks appear to rely extensively on composer for component integration. CI4 can use composer or not, and <spoiler alert> there will be a couple of other ways to use it in your app.