Yotsuba no Clover

It's the story of four sisters who are twins, and their names are Haruko, Natsuko, Akiko and Fuyuko. Each of them have their own special 'skills', like martial arts, high intelligence, etc. They run a detective agency, with everything in the firm taken care of by themselves, including the many times dangerous investigations. They do get customers, but somehow they keep on having serious money troubles.

(Source: MU)