My daughter is four years and 2 months old. About half a year ago she was asking a considerable amount of why-questions but currently, she doesn't. Is it natural at this age? I try to steer her in the math direction because it is my craft and I'm afraid that I'm too insistent and this inhibits her natural curiosity.
Children go through a developmental phase, around three or so, of asking "why". They do grow out of that - and four is a reasonable time to expect that - so it's not surprising your daughter stopped asking "why".
Part of the reason for that phase is that they don't have the vocabulary to really say what they mean; what they mean is, let's talk about this topic. When my wife tells me about some interesting scientific research she's doing, there are cues I can give her that say "I'm interested, keep going," even though I'm not a expert in her field and can't really use the right vocabulary to have a dialogue; but I like hearing her talk about her research, and like learning what I can from what she has to say, so I use my arsenal of "Hmm, why is that?," "What does that mean," "That sounds interesting," etc., to tell her that I want her to keep talking.
At some point, though, as your daughter learned more vocabulary (and details), she started to be able to say more accurately what she thinks. What you'll probably find is that she's no longer saying "why," but is still saying things that suggest she wants to talk to you about things.
At the same time, she also has begun developing more specific interests. Younger toddlers pretty much are interested in everything. As they age, they start to define their own interests. What may be happening is that she has gotten to the point where she can express her own interests explicitly - and talk about them. So at three, she may have been more interested in whatever you had to say; but at four, she wants to tell you about her things. That doesn't mean she doesn't like the things you talk about - it just means she now is ready to direct the conversation some on her own.
I know around that age, for example, my oldest son shifted from listening to me tell him about things as much, to telling me about the game he's playing or about trains or cars or whatever thing he was currently reading/being read to about. So the skill I had to learn was twofold: how to show interest in what he was talking about and encourage him to explain "why," and to identify whether he was receptive to me talking about my interests. He still was happy for me to talk about almost anything, but only sometimes - and I had to learn to identify those "sometimes".