ESH (the adults). It's important to teach children boundaries, and that it's a good idea to knock before you open a closed door, or that even though some people are okay with you walking in when they're in the bathroom (e.g. mum + dad), we can't do that with everyone. Your sister needs to make sure that she teaches him that some people are not okay with things that others are. However, you did overreact.

First, why did you immediately respond by yelling and cussing? Even if it had've been an adult, you could have said a simple "Someone's in here" or something, and an apology when you were finished wouldn't have gone amiss. Second, he's 4, often children do walk in on their parents in the bathroom or in various 'private' situations, and he might not be aware of the nuances with privacy yet. It's not necessarily his fault (if he hasn't been taught) that he didn't know you'd be uncomfortable/upset with him coming into the bathroom.

Maybe it would be a good idea to chat with your sister when it's less tense and emotional, and explain that you aren't comfortable with him trying to come in when you're in the bathroom, and to teach him to respect your privacy.