Your boards, as they flow through the water, provide lift in the opposite direction of the sail pressure to give resistance to the boat moving sideways. The center of the lift provided by the boards will be located roughly in the middle of the exposed board below the hull. If 4' of your board is extended below the water, the center of force is about 2' below the bottom of the boat. This gives it more leverage and means the boat is tipping over a lower center of force. If you extend your boards only 2' below the boat, the force is only 1' below the bottom and providing less tripping leverage.

The tradeoff is that you get less lift with less board in the water - however, there may be times when you don't care so much about lift from board...especially when sailing downwind. Downwind, you want just enough board in the water to keep the boat tracking in a straight line - sideways isn't fast.

Also keep in mind that this is a pretty oversimplified explanation - but is generally accurate.