What is MP4?
MP4 is the container video actually travels in - what phones record, what messaging apps accept, what every player since the iPod era opens. It usually carries H.264 video, and its one structural quirk matters to converters: the index of every frame lives in a table that has to be exactly right, or nothing plays.
What is WAV?
WAV is raw PCM audio with a small header - uncompressed, unprocessed and enormous, at roughly ten megabytes a minute for CD-quality stereo. It is the working format of audio editing, sampling and speech tooling, where every stage wants the samples themselves rather than a codec's approximation.