"You can tell the quadruplets apart by their hairstyle!"
Well unless they all oversleep and don't have time to style their hair. But yes, that was indeed part of why they did their hair differently like that.
Now we reached chapter 24, I can pretty much differ them without hairstyle
1. Ichika = Tallest of the Siblings, Has biggest bust, have much eyelashes
2. Nitori = Second tallest, Fang like tsundere, Sharp eyeline
3. Mifu = second shortest, Homely aura, Round eyeline
4. Shizuki = Shortest, Ahoge, Sleepy eyeline