WAV for audio work
WAV stores sound as raw PCM samples. It is the working format of audio editors and DAWs, of voice-over and podcast production, and of many speech-to-text tools. Extracting a video's sound to WAV is the first step to clean up noise, level the volume, cut a podcast or sync a soundtrack.
Lossless from here on
The audio of an MP4 was compressed once, when the video was recorded or exported. Converting it to WAV decodes that sound and writes it uncompressed: it does not lose anything more, however many times you edit and save it afterwards.
Sizes to expect
Count about 10 MB per minute (48 kHz, 16-bit stereo): 60 MB for 6 minutes, 600 MB for an hour. If the file is for listening rather than editing, MP3 at 192 kbps is about seven times lighter.