Mp3 files are required when using Springcast. Unfortunately, the files .wav and .mp4 cannot be used in Springcast, as these are not allowed with directories such as Apple Podcasts and Spotify.
It is not enough to change the file name from .wav or .mp4 to .mp3. You have to convert the sound file to an mp3 file. You can very easily convert .mp4 or .wav files to mp3 files via free tools like this one from Cloudconvert. Would you rather not use a free version for it? Then you can use paid version of, for example, Stellar.
So this way, you can easily convert your mp4 files to the mp3 file you want. It is best to export an episode to a 192-bit mp3 file. High-quality mp3 files are also very suitable for podcasts where music is streamed.