Even with a -4 it will be unlikely that an alliance between good and evil will form, unless the two nations have a common enemy. I fear that if the number is to great, then the chance of war between the two nations might increase too much. I'll playtest with a -8 and see how it works out. Let me know if you will want to playtest that as well, and I'll update the SVN sooner for you.
I did just have a thought. I think I will make the attitude value settable in the xml. That way, I don't have to recompile the dll every time I try a game balance change.