Hi Fierelier
Welcome to the forum.
I take it you are referring to ffmpeg4.tcz.
ffmpeg4.tcz.dep contains:
libavdevice4.tcz
libavdevice4.tcz.dep contains:
libavfilter4.tcz
libXv.tcz
libXfixes.tcz
libcdio-paranoia.tcz
libasound.tcz
SDL2.tcz
libopenal.tcz
libv4l2.tcz
libpulseaudio.tcz
libavfilter4.tcz.dep contains:
libswresample4.tcz
libavformat4.tcz
libavresample4.tcz
libpostproc4.tcz
libswscale4.tcz
libass4.tcz
So libass4.tcz should have been pulled in when you installed ffmpeg4.tcz.