4 years on and you haven’t let it go. I doubt you’re going to be able to let it go now.

Let her know how it made you feel. Be prepared with an ideal outcome. Example if you want an apology, let her know that’s what you want. If you want to put distance between yourself and your sister, let it be known.

This is going to be an uncomfortable situation to talk about. It’s very likely she didn’t see it as harassment so expect her to feel defensive. She might deny it. She might get pissed off. It’s possible she won’t even remember it or (for an optimistic option) maybe she’ll actually understand your perspective and apologise immediately but big sisters aren’t usually ready to admit fault. Just the natural hierarchy of sibling life. With you being the younger sister, you’re more likely to be mocked for holding on to that memory for 4 entire years. You should calmly stand your ground. You have a right to feel what you feel.

It’s important not to take offence to her reaction. It will take a moment for her to measure the seriousness of your words. Give her a moment to process. Reaffirm the seriousness of the problem if needed.

Be prepared to respond instead of react. If she says “you’re overreacting” - be ready to respond briefly and factually.

“I understand how it might seem like an overreaction to you but this is how I feel. I need you to apologise.”

You want to be in control and firm with your expectations but also calm. Keep it on topic and stick to facts. It’s a serious situation but refrain from making assumptions and going off topic or defining her as a predator etc. Go in with the intention to resolve it. Not with the intent to win an argument.

You might not get what you expect out of this but hopefully it works out for you.