Clicking ok takes me back to the preferences window. I click ok to confirm FFmpeg path and get this popup: 'Failed to find compatible FFmpeg libraries'. The path in the input box is '/home/user/apps/ffmpeg-2.0.7/lib/libavformat.so' (although I tried pointing to files ending with. I downloaded it, compiled with -prefix and -enable-shared flags, and located it in Audacity. The first one I tried was FFmpeg 2.3.6 'Mandelbrot'. I started looking for ffmpeg sources with avformat version 55 here. 'Locate FFmpeg' window tells me to find libavformat.so.55. Nothing weird still, I may have an incompatible version installed on my system. A popup shows up saying 'Failed to find compatible FFmpeg libraries.'. So I go to Edit > Preferences > Libraries and click on FFmpeg library 'Locate.'.

Otherwise, you need to convert it to a supported audio format, such as WAV of AIFF'. Without the optional FFmpeg library, Audacity cannot open this type of file. Importing an mp4 file gives me this error message: Audacity 2.4.2 (installed from official Fedora repos)

Open mp4 audacity ffmpeg