Four is a bit young for a sleepover, but we love the idea over at Sugar City Journal to have a pretend party complete with hand delivered donut invitations. So cute!
The girls got to do all the fun sleepover things without the sleepover: wear their pajamas, eat a donut cake, paint their nails, and (not sure we did this at our childhood slumber parties, but sounds like fun for a four-year-old!) toilet paper a bush in the yard!
More details on the fake sleepover birthday party on Sugar City Journal.
(Image: Sugar City Journal)

White Enamel Flatwa...
Here in Milwaukee, we call them "sleep under" parties.
With 3 kids we have had to make adjusted accommodations for the younger two when they would like the privileges the older/oldest kid has. When #2 wanted a friend over for a sleepover at a too-young age we staged an "almost sleepover" with dinner and pj's and a movie and special snacks. At 10 pm-ish, the parents came and took the sleepy child home. Bonus: they got a dinner date in themselves. Now #3 is lobbying for his "almost sleepover" event!
My four year old not only has had sleepovers in our home but she has already been to two sleepovers herself. I'm always prepared to have to go and get her at whatever time she may want to come home but as of yet it hasn't happened.
When we host sleepovers it's usually two other kids (a little boy and a little girl) and we make a big fort in the living room and the kids stay up late eating popcorn and cookies. =)