It really depends on how far out of the typical development of a 4 year old this hyperactivity is, and, if you see other issues that give you a concern. It is not true, btw, that kids cannot be diagnosed early, they can, and should be, if they have an issue, and even if the official diagnosis is not given due to age, children with issues can get early intervention for the issues that they have, which is the very best thing any parent can do, if, in fact, the child is not developing in a typical way.
You would be seeing an inablity to carry out multi step directions, difficulty aplying typical dicipline applied consistently to themselves, meaning, if you are consistent with any good diciplinary technicque, and they know a concequence is coming, and they still cannot manage to change the behavior at all, then you have a behavioral concern that may be neurological. Contrary to popular myth, this issue is better if parents seek the earliest possible intervention, and not wait until the child is school aged and has internalized this missbehavior as part of thier everyday life. It is far more difficult to overcome if it festers. Neurological issues are there prior to us knowing that the behavior is not going to improve, so that sooner you get to it, the better.
Other issues you might see are the inablity to sit and do fine motor tasks, not being able to sit and listen to a book, or not being able to stay focused with any one toy or game to the end, which begins to damage thier ablity to make and keep friends.
The big one is Impusivity. If your daughter cannot stop herself from doing things, even when you have practiced, and you have workded on it, and you have applied concequences, and it is obvious to you that in a group of kids, the others are (while not free of impulsivity, because impulsive behavior is typcial for children for many years to come) better at impulse control than she is, and have many fewer impulsive behaviors, or that her impulsiveness puts her in danger and interferes with her social development, then you may have a developmental issue that is consistent with ADHD.
Since all you mentioned here is hyperactivity, I am hoping that this is just a kid who likes to be active, and you might try to give her more physical opportunities that are appropriate. Maybe a personal trampoline (like for a work out) in the family room that she can jump on that is an approved form of physical activity in the house to get the wiggles out, or put her in soccer, or have her run around the back yard in laps and race with her brother. Give her a hug if she lets him win. Something to channel the activity, and if that does not help, and you do see her in the other things I mentioned, an evaluation is always a win-win.
I mean, if you take her to see a professional (Developmental Pediatricans, psychiatrists w/a fulll neuropsycholgical eval) then you walk out the door knowing that you either have nothing to worry about, or you walk out the door with a plan to help your daughter be all that she can. Win, win, and nothing not to like about that.
Good luck
M.