You need to edit the table qa_posts in the mysql database. The content field has a maxlength of 8000. Sounds like you may need to learn how to use mysql... phpmyadmin is an easy tool to do the job.
EDIT: in phpmyadmin:
open the qa_posts table, click the Structure tab, find the "content" entry, click "Change", find "Length/Values" and see that it is at 8000. Change that, click save.