This one is rather fun for a tank. Four towers, one flag.
The general strategy is to leave the flag alone and capture towers ASAP. A semi-decent team will have two smaller groups capture the two towers closest to your starting spawn, and either one massive or two smaller zergs fighting for the tower/s closest to the enemy starting spawn.
For a tank this presents a golden opportunity to do it all wrong, provided that you clearly state what you're up to before the battle starts. You're hitting the flag, and you don't want any help.
Most often this is a suicide mission. You get to the flag and blow every defensive CD in your arsenal in order to keep enemy players tied up by a flag they shouldn't have come to in the first place. If you can keep four or five players busy there for some thirty seconds, then your reward is likely to see three towers captured in the absence of defenders.
Sometimes the enemy team has a working leadership, and you'll end up by the flag alone. Not much to do about it. Collect and head back home to capture it. The lack of a single tank at the towers is outweighed by the extra points you get by capturing the flag.
If you're followed by a bunch of teammates to the flag, just ride through the area and attack an enemy tower. You're up for a bad start with a group of people hitting the flag instead of the towers, and the best way to do something about it is to go suiciding at one of the enemy towers in the hopes that your presence there will see the enemy team move more players to defence. It's unlikely, but I've seen it happen.
After the battle has started in earnest you should rotate between three types of duties. Defend an undermanned tower, attacking an undermaned enemy tower or hitting the flag if there is an enemy group at it.
If you're defending, do call out incoming enemies. Your job isn't simply to suicide. Your job is to stay alive until reinforcements can arrive.
If you're attacking a tower alone, then you are indeed suiciding. Make an effort to get inside, or behind a corner, or whatever you can do to move the defenders away from a position where they can see incoming forces. Chances are they'll call out for reinforcements to a defence which really doesn't need any.
If you're hitting the flag, then you're suiciding to keep enemy players busy at a location they shouldn't be in.
Sometimes you can even help your team by fighting on the roads. This will only be true if you can grab the attention of three or more enemy players.