A 4 year old isn't a toddler anymore mom...she's a preschooler now. She's growing up.
You don't say what time she's going to bed, what time she gets up, what sort of sleeper she is normally.
If she's been going to bed pretty early chances are she's just not needing that much sleep anymore. Keeping her up later will probably fix your issues.
Also she may just be going through a stage and in the next few weeks she'll be right back to normal.
My grand kids each and every one, as an infant, would get up around 7:30-8am and take a morning nap then an after noon nap then an early evening nap then down for the night right around 9pm.
About the time they turned 1 they were down to one nap during the daytime hours then when they got picked up from child care they'd nap on the way home. Once the get about 10 or 11 months old the infant room teacher starts getting the kiddo down to one nap per day. That way when they move to the toddler room they'll be on that classroom schedule. When they turn 2 and more to the preschool classroom they take a little bit shorter nap and so forth to the 3 year old preschool classroom and the pre-K classroom with the 4 year old kiddos, this age kiddo takes about an hour nap.
If they take a short nap in the early evening they don't need to go to bed until later in the evening so they'd stay up until 9. Our kids go down nearly every evening easily and don't have any issues very often.