The next big step: Godot 4.0 reaches Beta, Release candidate stage now.

cybereality I haven't noticed any conflicts. I have got 3.x and 4.0 installed for a while.

After backing up the editor data, I briefly tried 4.0-beta and saw that it changed some existing files and added a lot of stuff. That may not affect functionality, but I decided to revert the data from the backup, and use 4.0-beta in self-contained mode. I think that's safer and cleaner.

I came across one aspect of self-contained mode that was unexpected. The app_userdata directory in the common location is still used for project data. So if you're testing a copy of a 3.x project under 4.x, you need to change the name of the copied project, unless you want the two projects to share the same app_userdata directory. This can be done by editing project.godot before accessing the project with Godot, and changing the value of config/name in the [application] section.

This is evidently working as designed, as seen in this old issue:

akien-mga commented on Oct 14, 2017
Note that app_userdata is not meant to be local in the self-contained mode, that mode is for Godot itself but does not impact Godot games. You should be able to access the same game data from a system-installed Godot and a self-contained one.

https://github.com/godotengine/godot/issues/11813#issuecomment-336622952

I submitted an issue. It's not a big one, though.
3-to-4 project converter adds extra line for move_and_slide #66054
https://github.com/godotengine/godot/issues/66054

I also encountered a couple of other small issues with the project converter, but I don't know if they qualify as bugs:

1. An instance of "Reference" in a comment was changed to "RefCounted".
2. The values of the settings Project >> Project Settings... >> General >> Display >> Window >> Viewport Width, Viewport Height were changed.

Do you think those should be submitted as issues?

I submitted the first one.
3-to-4 project converter changes instance of "Reference" in a comment to "RefCounted" #66061
https://github.com/godotengine/godot/issues/66061

Regarding the second one, I noticed that in 4.0-beta1, there are two sets of dimensions: Viewport and Window. The Window dimensions were preserved. It's the Viewport dimensions, which appears to be a new setting, that differs. Maybe that's not a bug? Or should the Viewport dimensions be set to the same values as the Window dimensions when converting a project?

[deleted] Can we fast track working HTML5 export in Godot 4 ?

You might ask about that in the Contributors' chat room to get the most accurate feedback.
https://chat.godotengine.org/

Here are some fixes I had to make to a small project, that the 3-to-4 project converter didn't (couldn't?) fix. Do you think these are things that the project converter should handle?

Or should these be added to documentation (somewhere) for reference by others when converting projects? Figuring out these fixes can be time-consuming without a conversion guide, since you may need to use both the old documentation and the new documentation. For example, I had to use the old documentation to locate the class (CanvasItem) for which update() was a method, note its description, and then find the corresponding method in the new documentation.

1. CollisionObject2D._input_event() signature changed: first parameter type changed from Object to Viewport. This only required a change since I use static typing.
2. CanvasItem.update() renamed to CanvasItem.queue_redraw().
3. String constructor String(int) removed; use str() instead.

Update to my previous post: Calinou said in the contributor's chat that #2 and #3 would not be feasible for the converter to handle.

4.0 Beta 2 has been released.
https://godotengine.org/article/dev-snapshot-godot-4-0-beta-2

Too late ...

Looks like the build targets were changed. There is editor, template_release and template_debug now. Size of the editor executable is consistent with having no or very few debug symbols.

Would be cool to have a build with debug symbols, if devs want bug reports ...