RE: "whether the global keymap C-x 4 will be replaced by a command,"

[Drew Adams]

> The least work would be to just add a "C-h 4 C-h" command to get help
> on the "C-h 4" commands available as is done in many other places.

`C-h r C-h' already lists all of the `C-x 4' keys
and their commands.  And choosing any of those
then shows you its complete help.