5. Well, if you ask her "Five and a half." And it's not really "hers", although she's the only one who uses it as a primary machine.
My husband and I have a computer consulting business, so both of us have business laptops. This year I needed a new laptop, for various reasons.
So I got a new laptop, and the old one became the business "spare", in case one of the business laptops has a hardware problem. It's also the business "guest" machine, for when a non-working meeting with clients/consultants turns into a working meeting. It's the projection machine for working meetings. It's the air travel laptop, the guinea pig, and on and on.
And for personal uses it's our multimedia store/DVD player/streaming video player/etc. DD has an account and considers it "hers". She checks and sends email, does her little web games, typing tutor, watches DVDs etc. Massively more computing power than any 5 year old needs, but she's taking advantage of time it would otherwise be idle. And when the business needs it, she logs out and we log in.