Bithionol is an antibacterial, anthelmintic, and algaecide. Bithionol (the compound is also called bithional) is added to soaps to impart antiseptic properties. It is aryl sulfide that is diphenyl sulfide in which each phenyl group is substituted at position 2 by hydroxy and at positions 3 and 5 by chlorine.

So, as we can see in the structure it has total of 4 chlorine atoms.

Chlorine atoms $=4$