Seems like everyone knows best how to be a parent here. I bet nobody is yet.

To answer your concerns:

A) your brother is 4 years old kid. Unless he has been exposed to porn or anything like that this cannot be classified as an assault. You may be jealous or uncomfortable because your brother is a little kid and needs attention from your parents. This is understandable but you have to be in their shoes. Now: you being in a bath with your little brother it's very normal as he probably loves you and wants to have fun ( play) like kids do in the bath with his sister. You are more than capable to cover your sensitive parts with bikini and that will make your situation a lot easier.

And I do believe you are over reacting. It's your brother not some foreigner who asked you to bath with you. Show some compassion and empathy. It's your blood, stop being jealous and help your parents to grow him and you can be part example how he will treat ladies in the future. Educate him and love him as a family member. Enough drama with all the fearful experts in this section. Lol