Is she able to make it until bedtime or does she meltdown because she's exhausted? I wouldn't expect to get 3-4 hour naps from her at this age unless she's in a growth spurt. But an hour to two isn't unusual or unreasonable. Not having a nap routine and letting her be up sometimes and trying to reintroduce nap sometimes doesn't usually work. Kids learn that if mom lets them be up sometimes, they can be up all the time, and no matter how tired they are they won't go to sleep.
If you don't have a nap or quiet time routine, I would adopt one, by brushing teeth and settling down with some quiet stories with her (outside of her room), then taking her to a dim room and giving her some favorite books to look at or rubbing her back for a few minutes. If she's not focussed on *fighting* sleep she may relax enough and fall asleep anyway... And I agree with Abby, after an hour if she isn't sleeping, I'd get her up.
Some kids outgrow naps by this age or are in the process of it. I think down time is still vitally important though. There's probably a degree of control in it for her, so I would let her know that she doesn't have to "nap" but she does have to have quiet time in her room.
Good luck!