The phrase is from an animated tv series called Teenage Mutant Ninja Turtles theme song. You can see the full lyrics here: https://genius.com/Teenage-mutant-ninja-turtles-turtles-theme-2003-lyrics
This is where it first appears:
Teenage Mutant Ninja Turtles
Turtles, count it off!
One, Two, Three, Four!
Wikipedia says it's something related to music to establish the piece's initial tempo, time signature and style. But this doesn't appear to be related to music at all. Is it?
Perhaps it's just because the main characters are four? Or maybe because they follow four rules? The four rules from the lyrics
One! / Live by the code of the martial arts.
Two! / Never fight unless someone else starts.
Three! / Always stick together no matter what.
Four! / If all else fails then it's time to kick butt!
Dictionary.com had this entry:
Count aloud from one end of a line of persons to the other, each person counting in turn. For example, The soldiers counted off one by one. This usage and the practice it describes come from the military. But what is