[fishBurger]

wow. This map looks like it is going to be awesome!

For the fourth fort, I see something with perhaps an industrial theme? Doesnt have to be the generic warehouse or the somewhat-overused grungy abandoned industrial area, although either would work if you wanted to use them.
I'm seeing perhaps a well maintained manufacturing facility or maybe a sawmill type deal.

As far as gameplay, heres an idea. 4 flags (or objects map/team specific). If you toss your flag or object into the cap area of another team's base, you get 10 points, they lose 5, AND I like the idea of having that trigger a blocked access (main exits slam closed for 20 seconds) or something fun for the attackers (base sniper deck explodes into flames) or something.
now heres the fun part: any team can grab any flag, so you're trying to push your flag into an enemy base while denying the enemy access to their flag. If you toss an enemy flag into their own cap zone, then you dont get points, but they lose 20 or 30 and ALL their base exits slam closed for 15 seconds... now when they open again that entire team is massed at the door, ready to break out in force and cause some damage.

the whole scenario provides so many opportunities for teams to team up and work together, and just as many to backstab the other guy once you've wrecked another team's base.