Formal:
* J'ai quatre frères et sœurs. (This is the most literal translation.)
Informal:
* J'ai quatre frères et sœurs. (This is the same as the formal version, but it is more commonly used in informal contexts.)
You can also use the following phrases:
* J'ai quatre frères et soeurs. (This is a more common way to say it in informal contexts.)
* J'ai quatre siblings. (This is the English word "siblings" directly translated into French.)
It's important to note that "frères et sœurs" literally means "brothers and sisters", but it is commonly used to refer to "siblings" in general, regardless of gender.