Children with MILD emotional problems don't disrupt from 4 adoptive placements. This girl likely is conning the case worker which is preventing her from getting the care she needs. She needs to be working with a counseler who understands adoption, foster care, and attachment as I doubt this girl has any trust for anyone at this point.
If you cannot love this girl and parent her if she never changes, don't. A few small fires are a big deal and should not be written off as a stress of any sort.
I would recommend reading Foster Cline's Can This Child Be Saved? if you can find it and looking at
www.radzebra.org and
www.attachment.org.
With just the information you've given, I would suggest putting off the finalization date until your sure, disrupt if you feel that your mind is made up, and request subsidy for future hospitilization and residential care if deemed necessary. If you decide to continue, an attachment therapist is needed.