I think it depends on why you think she's asking and what you think is important. Often, when they ask, they already know or suspect. But sometimes they don't - they're just making a list! And of course we always tell kids that monsters and ghosts (whatever they're afraid of) aren't real.
But you've also got the problem of her running around the preschool telling everyone that X and Y aren't real. That might be okay with superheroes but it's going to cause problems if she says that about Santa, the Easter Bunny, and the Tooth Fairy. (We're Jewish and never had Santa or the Easter Bunny come to our house, but I sure didn't want my kid to be the one who burst other kids' bubbles! So be careful before you start down that road.
I do think you can put it back on her and ask what she thinks. I think it's okay to talk about why people believe in superheroes - because they want the good guys to win over the bad guys, they want someone who is strong when they feel weak, and so on. Look around at the heroes we have in everyday life. You don't want your kid thinking she can fly like Superman and decide to jump out of a tree, but you don't want her afraid all the time either because she feels vulnerable and unprotected (as any child would).
Have you read the story that's going around about what to do when a kid asks about Santa? Google it, but the gist is that the parent first asks the kid what she thinks, but if the kid persists in saying Santa is make-believe, then the parent takes the kid out for hot chocolate or lunch and says, "You're now old enough to BE the Santa instead of getting from Santa, and that means doing for others and helping others younger than you to enjoy Santa as you always did." So you might pursue, in an age appropriate way, what she admires about this hero or that one, ask why those powers might be important, and ask in what ways we humans can be a little more superhero-like in our daily lives. You'll do that in a different way at 4.5 than you will at 6 and 9 and 12, but you get the idea.
You can also ask your children's library staff (the most underutilized resource in any community) for some books that help kids get through these transitions from fantasy to reality. Those might help you, and might help your child.
I think it's also okay to look at her favorite books and point out that they are fiction but totally enjoyable. Dr. Suess characters don't really exist, but don't we love the stories anyway? Don't we love Winnie the Pooh even though bears don't talk and don't have friends who are piglets and kangaroos? Don't we learn something from the way they all get along together? Do we play dress-up and pretend to be dinosaurs or queens or grown-ups, and is that all bad? You see where I'm going with this.
Good luck!