How should I explain death to my four-year-old?

No one has died, no one we know is very sick and dying, but recently she has put all the pieces together and has figured out that everyone dies, especially old people. She was crying this afternoon, saying she didn’t want to ever have babies, because that would make me a grandma and grandma’s are old and old people die. I don’t think she even knows what dying really is, just that it’s sad. All I could do at the moment was hug her, stroke her hair and say things like “Oh, sweetie, it’s okay,” and “Honey, I’m still going to be your mommy when you’re grown up!” But I know that those things didn’t address her fears. What should I say to her? Should I wait for her to bring it up again, or bring it up to her?