Download best mp4 format available or any other best if no mp4 available
youtube-dl -f 'bestvideo[ext=mp4]+bestaudio[ext=m4a]/best[ext=mp4]/best' URL

Download best mp4
youtube-dl -f mp4 ...

List-formats
youtube-dl -F ...

Download specific
youtube-dl -f 22 ...

Download the best video format and the best audio format without merging them
youtube-dl -f 'bestvideo,bestaudio' -o '%(title)s.f%(format_id)s.%(ext)s' ...