HI M.,
It sounds like the daycare is a good fit for her now and I would let here stay there. Having had my kids in a daycare until a year ago and now having a daycare in my own home, I really see the benefits of having kids in a smaller environment when they are younger if you have a good provider.
I think you'll know when she's ready to transition out as she gets a little older. Usually what happens is that the kids in the daycare get "too young" for the older kids as the kids transition in and out and the older kids are no longer challenged in the same way as they were when their peer group was close to their age. Then there is great benefit to preschool. They interact with their same aged peers, do age appropriate activities and usually have more structured activities.
I would HIGHLY recommend looking at preschools now, however, and perhaps getting on a waiting list. We live in an area where preschools are impacted and there is a long waiting list ( 2 years for the a.m. program that my son will be attending in the fall). You want to be able to have options and if your daughter comes up on the wait list and you aren't ready to place her then you can decline. I would ask the preschools how long their wait list is and what their deposit cost and policy is. For some, if you decline the spot when it's offered, you don't get the deposit back, but it's better than your daughter being ready and not having an option for a program better suited for her.
If you are considering a half day preschool option, I would apply for an a.m. slot as your first option. In our situation our daughter was still napping at age 4 and it was a difficult transition for her to have to miss her nap. Some kids are ready to give it up by then, but she still (at now 5) is fried by the end of the week. But she is a busy girl also. So it depends on the child. My son, who is also very active, still naps at age 3.
Good luck~ hope this helps!
J.