When she get to an age when she no longer believes in Santa, the Easter Bunny, the Tooth Fairy, and Fairy Tales then you can help her question what she has been taught. At the age of four she is too young to be caught in a tug of war.

Perhaps just tell her "That's not what I believe." or "That's not how I feel." and see where it leads. I don't know how old your younger sister is but she may be able to suss it out for herself.

I have two young great nieces who attend a private lutheran school. They get a huge dose of religion every day. Their parents have the right to raise their children as they feel necessary, without worrying about other people trying to create confusion or dissonance. I keep my views to myself. When the oldest niece was five, she told me that she had learned at school that those who were not believers are "lost," and then she asked me if I believed as she did. I deflected by saying that there are many people all over the world who believe many, many things differently than she does, and that they don't seem lost at all. That is the extent to which I would engage a child in such a discussion. If she wants to discuss it when she is in her teens, we may go a little further with it.

In principle, there's no problem mentioning your own [un]beliefs, but if your sister is in an indoctrinating / controlling / dogmatic / fascistic sort of thing like fundamentalist Christianity, it may drive her away to do so.

I would just be kind and authentic. It's that easy, and that hard / risky.

God is a fiction, why beat around the bush?

I'll go with a popular sentiment here - don't say "God isn't real." Instead, say something like "I don't believe that God is real." And then explain why you think that. Ultimately, yours is just one opinion in a sea of opinions. Are you right? Yes. No. Maybe. Who the hell knows. The point is, present your view as your view. Your sister has to work this shit out for herself. You won't do yourself any favours as coming on strong; instead, I would suggest being the calm, reasoned and rational voice which acknowledges its own perspective and lets your sister make up her own mind.