What is actually inside an MP4
An MP4 is a container: a video track, an audio track, and an index describing where each piece sits. Converting to MP3 means throwing the video away, decoding the audio track, and encoding it again as MP3. Almost every MP4 stores its audio as AAC.
That last detail is worth knowing, because if you do not specifically need MP3, choosing M4A instead skips the decode and encode entirely and copies the AAC across untouched. It is faster and it loses nothing. MP3 is the right answer when the file has to play on something old or unfussy.
Which bitrate
192 kbps is a good default. Speech is fine at 128 kbps and you will not hear the difference; music you intend to keep is worth 256 or 320 kbps. The estimate above the button updates as you change it, so you can see the size before you commit.