Maybe you approach it from the perspective of "now you're getting to be such a big girl! Next year you'll be able to go to school like the big kids" and talk to her about the fun parts of school (spare the agony of math class for a later time!).
Find out what this all-day preschool does and describe it a little to your daughter. For example, do they ever go on field trips? Do they have a cafeteria or lunch room? Will your daughter be bringing a lunch from home? If so, take her shopping for a new lunch box. Do the preschool kids get to do art projects, or learn about different subjects at activity centers, like nature or some simple science? Is there a class pet (a gerbil or whatever)? What do the teachers call the rest time in the 4 year old program, if there is one. They may call it quiet reading time, or quiet time, or some other name, and you'd want to use the same terminology.
Then you can say, with enthusiasm, something like "when you go to school next year you'll be a four year old, and they don't come home for naps. The four year olds get to [whatever is included in the 4 year old program] and it will be exciting!" Don't really spend much time discussing naps. Instead, focus on the pride of being 4, and talk about how much fun kindergarten will be the year after that.
After all, it's a normal progression, and kids don't usually feel like they're being kicked out of the home just because they're now old enough to attend school for a regular school day. True, your all-day program is still voluntary, but it's not that unusual for a 4 year old to spend the regular school hours at school.