Lots of Internet says to "add-apt-repository ppa:jonathonf/ffmpeg-3" for good ffmpeg, but don't if you're bionic (ubuntu 18). Instead, press "add-apt-repository ppa:jonathonf/ffmpeg-4". Also probably remove the old one with "add-apt-repository -r ppa:jonathonf/ffmpeg-3"
... and if, in a moment of weakness, you installed ffmpeg to /usr/local, nuke that, and don't forget the libs! If /usr/bin/ffmpeg keeps barfing, search and destroy the results of "ldd /usr/bin/ffmpeg | grep "/usr/local/lib"