I am gonna' go out on a limb and say - Please never use police officers, doctors, firemen, etc. as threats to get your children to behave. These are people and professions that our children need to trust. If she is now frightened that the police think she a "bad person" then she will not go them in an emergency...yes, she will outgrow this but really, don't use an entire profession as a threat to get your child to follow the rules.
That being said, you should have made her return the bookmark and explained to her that she should not take things that are not her's without permission. This is a simple life lesson for a 4 year old that can be accomplished without instilling fear of our public and professional servants.
You need to begin having discussions with her to mitigate her fears...when she questions the cameras start explaining that they are there to help the police protect her and her friends. Continue to explain that she is not a bad person, but that her actions were "bad". You can work through this with her.
Good Luck