Philipp Stephani <p.stephani2@gmail.com> schrieb am Di., 4. Apr. 2017 um 14:22 Uhr:

M-x ido-mode

Then try
C-x C-f
C-x d
C-x 4 f
C-x 4 d

In the first three cases, IDO will be used, but not in the fourth case.
That's a weird inconsistency.

Here's a patch (which includes a couple of other commands).