I don't think the question is "what is normal" because that is a transient thing.
The question is how do YOU feel about this and what message would you like to send to your child? Also, what is the role you have in her life, if she is your step child, are you supposed to be letting her bio parents make the decisions for her? Because if that is the case, then your options are severely limited.
Personally, talking about sex and sexuality, for us anyways, is an ongoing conversation. My son (age 1.5) knows what his penis is and how to say it. He knows that he urinates with it and we talk about that. When I wash him I say the names of all of his body parts. I also talk with him about PRIVACY and how his penis is a PRIVATE PART. When he touches his penis I say "oh, that's your penis!" in a totally calm way, as if he had brought me any other thing and I was identifying it for him.
DD is 3. She knows her body in a very intimate way. She knows she has a vulva, she says the word. She is aware that she has a urethrea which she pees out of and a vagina, which is where a baby is born. She also is aware of her clitoris. Sometimes I have happened upon her feeling it and laughing and smiling and I say, as I mentined before "oh, that's your clitoris."
Now, I also stress with her that this is her Private Part and that this is something that just belongs to her and it's just for her to see and touch and not anybody else. No shame about it, but a sense of holiness about it. I also try to model things like when her brother is on the toilet, I try to tell him to push his penis down into the toilet before doing it myself (he doesn't get it yet) I will also add, when ds is old enough he will do that himself cause that is HIS private part and it's not even for Mommy to touch.
When she is with her friends and they both have to pee outside or something like that, I stress that they are PRIVATE PARTS again and try to encourage modesty by having rules like "no leaving the house without clothes" or "no running around the house without your underwear. WHen dd asks why, I tell her because these are her private parts and we need to keep them covered for modesty because they are private. I also tell her that they are very special parts that we must keep very very clean and underwear protects them from the dirt and other grime from outside and inside that she might sit on or encounter.
When she is reaching down her pants, I tell her that she needs to wash her hands both before and after because it is an area that harvests bactera easily and we need to keep it clean.
Of course all of this is true, but I"m also trying to teach her (and my son) that these body parts aren't like the others and special care and consideration must be taken when they are involved.
dd has asked if she can touch ds's penis...washing it or changing his diaper. I've told her that she may not because it is his Private Part, that only I can do that and as soon as he is able (like she is) to deal with bathroom (washing, using the toilet) himself it will be ONLY him who can touch that.
So I suppose I would start there for a young child. But at this point, I don't think that those conversations are remiss. Espically after the molestation, which I'm not sure how you handled, but presumably a lot of this was dealt with then. About how these are Private Parts and how others can't touch them and we shouldn't touch others and only later on when there is a certain level of sophistication do we start talking about sex and then you can frame it as you wish. For our family, that will be something like, "one day, when you get married..." and it will be framed in that way. For someone with different ideas about sex that might be framed differently like "when you feel ready" but of course, I don't agree with this lifestyle choice, so I wouldn't be the one to comment on it.
At any rate, I think until that time, and certainly age 4 for a previously molested child, there would be a much more delicate way of handling the situation using words, books, pictures, and really stressing the holiness and importance of these very private parts.
So in summation, I would say, decide what your values are. And then respond accordingly, provided you have a say at all with a step child.
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