So, update has come along for ffmpeg 4.
wine-any currently already requires it at least (others might too, unsure).
Problematically, its an ABI break.

I've pulled in the gentoo ffmpeg-4.0.2 package temporarily to test.
But this also requires mplayer and alsa-plugins (at least) to be patched, otherwise they fail on 'av_resample' and other undefined references.

Patch in gentoo main repo for mplayer fixes one part of this. (ffmpeg4 patch in mplayer/files/)
alsa on the other hand might be harder, but i haven't looked much into this part as it might be easier to bump alsa version for compatibility.