Mark,
A few quick things....
(1) I'd take your daughter to a mall with a Best Buy, Apple Store, or anything like it that has computers....just to get an idea what she wants. I know a lot of young girls really like Netbooks (the little 10-inch mini computers) because they are so small. They aren't great for photoediting, but they do other 'basic' tasks (email, internet, photo viewing, movies, word processing, etc.) very well. Plus, if your daughter likes 'em, they are cheaper! 
Only downside here is I'm not sure whether it would be good enough to keep her happy for 4 years!
(2) As others have mentioned, if you decide to go for a full laptop....I'd definitely eye something with an i3 or i5 processor. That's the *current* generation of processors that have become pretty ubiquitous now over the last 6+ months on the market, translating to pretty good deals.
If 4 years is a major concern, I'd look at the website called Newegg.com....and then look for laptops made by ASUS. They are one of the best PC companies out there right now, and most of their laptops come with a 2-year warranty (as opposed to the typical one year!). Newegg has an add-on 2-year warranty (for a very modest $60-80)....so you're covered for 4 years.
That's important because most low-end laptops (that would be perfect for your daughter otherwise) aren't really designed to last that long. You're doing pretty good if you get through 3 years on most of these....but 4 years is definitely pushing it!
Of course, if you expect your daughter will do something dumb like lose it OR spill a Coke on it, then the basic warranty's are out the window....haha!
(3) If your daughter decides she wants a Mac and the father decides she can have one, I'd *definitely* check out the Apple Refurb store on-line! Pretty good savings over a newer computer. Something like a low-end Macbook would be perfect for a high school/college aged girl. I've used Macbook Refurbs as my primary work computer, and it does awesome! But the deals depend on timing....sometimes the deals just aren't there.