couper les cheveux en quatre

to over-analyse minor details

This expression means to nitpick or over-analyse something to an absurd degree. Similar to the English "to split hairs."

On ne va pas couper les cheveux en quatre, l'essentiel c'est que ça marche.

Let's not split hairs, the important thing is that it works.

The original 17th-century form was 'fendre un cheveu en deux' (to split a hair in two). By the 1798 Académie française dictionary it had become 'fendre un cheveu en quatre,' and the modern 'couper les cheveux en quatre' settled in during the early 20th century. Furetière noted it in 1690 as meaning 'to over-subtilize.'