* "Quatre sœurs" is the most direct translation. It literally means "four sisters."
* "Des sœurs, à quatre" translates to "sisters, four of them" and is a bit more descriptive. This phrase is good for emphasizing the number of sisters.
* "Un groupe de quatre sœurs" means "a group of four sisters." This is a good option if you want to focus on the group aspect.
So, depending on the nuance you want to convey, you can choose any of these options.