I'm looking for ideas for an educational toy for a 4-year-old girl. What factors should I look for?
I'll try to answer, although the question might get rephrased:
You need to take several factors into account. Consider these aspects, and they might help you narrow down your quest into something that is specific enough that you can search for items on Amazon and read some reviews.
Perhaps a small easel or blackboard is the thing. I think it encourages artistic play. I've also seen lots of wannabe-laptops for kids which I personally wouldn't buy, but obviously many people do. You could even be very advanced and get the child a small real laptop (a netbook costs around €250 these days) with childrens' programs on it (I think Ubuntu is a Linux flavor that is very well suited for this).
I do encourage you to clarify your question - perhaps this is way off the mark.