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

[Richard Stallman]

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  > It also affects `C-x RET c`, which is another standard prefix command.

The effect of that modifier is totally uniform.  We should not think
of it as like a prefix command.  It is not similar to this issue.

A numeric argument is also a uniform modifier.  However, plain C-u is
a fishy halfway case.  Because we have made so many commands do
something different after C-u, semantically it makes sense, sort of,
to think of C-u WHATEVER as a different command from plain WHATEVER.
That is why that case is confusing.

-- 
Dr Richard Stallman
Chief GNUisance of the GNU Project (https://gnu.org)
Founder, Free Software Foundation (https://fsf.org)
Internet Hall-of-Famer (https://internethalloffame.org)