Books that include these topics:
Everybody Poops
Once Upon a Potty
Some of the children's books you are familiar with have characters that need to pee and poop inside the story. Ask a librarian which books to choose - she may respond to book "friends" who use the potty.
I have a friend who's daughter would not poop unless she had a diaper on. She started to change this before she went to kindergarten and realized that her mom would not be there to put a diaper on her each time she needed to go.
It is possible that PK will do the trick by itself, but if not, she may have to wait a year to go to school. It takes a ready psyche to realize that no one will be there to help you once you are in school.
Alternately, it could also backfire on you if you push her to use the potty so that she can go to school (especially if you tell her that YOU won't be with her in school). She is developmentally able at this age to put something off because she is afraid of the separation that school will bring - so you can't count on the desire to go to school as an incentive or a threat at the age of four.
I am guessing that an adopted child already has separation issues ~ tread delicately on this one. She is probably loathe to separate from her own pee and poop. Many children, adopted or not, have issues with this most basic separation.
Get her some books, make it fun and cross your fingers.