multiple forums logging in query

I have four separate forums and wonder how I can make it so that after logging into one of the forums a member can click on the page of another forum and not have to relog into the second forum.

All forums will have the same user names/passwords etc.

And I also want to have say three forums using the same database and I know I have to make the new tbl prefix for each table in the database but is it possible to hardcode the second and third forum (which files?) so as to lookup member details from the same table. ie all three forums have their own table prefix and tables for things such as the topics, posts etc but all three use the same Author table?

ok thanks Borg, I am wary of getting into database difficulties so I will take your advice and keep teh databses separate.

That means I need to enable users to move between the different forums without having to relog. Any ideas on that one?

I have read many posts on the topic but cant seem to get my head around the  answer yet.