Although media-video/mplayer-1.3.0-r5 compiles w/ ffmpeg-4 (but see bug 655442) the mencoder program built is practically broken since it completely lacks support for lavc.
For the user this manifests as described here:
The only reasonable workaround seems to use a newer svn snapshot of mplayer
(see also the above link).
So... time for a snapshot?
The bug has been closed via the following commit(s):
Author: Alexis Ballier <email@example.com>
AuthorDate: 2018-11-06 11:20:15 +0000
Commit: Alexis Ballier <firstname.lastname@example.org>
CommitDate: 2018-11-06 11:20:34 +0000
media-video/mplayer: backport upstream fix for ffmpeg4 + mencoder.
No revbump as ffmpeg4 is masked.
Package-Manager: Portage-2.3.51, Repoman-2.3.12
Signed-off-by: Alexis Ballier <email@example.com>
.../files/mplayer-1.3-ffmpeg4+mencoder.patch | 360 +++++++++++++++++++++
media-video/mplayer/mplayer-1.3.0-r5.ebuild | 2 +-
2 files changed, 361 insertions(+), 1 deletion(-)