ONE, 4' X 6'-6" dormer will cost you about, with labor and material, $2200. So, you want four, then your looking at about $10K.
One major thing to consider, you will have many valleys and use alot of flashing for the roof surface. Get a very good roofer and one that uses alot of flashing and protection on the valleys. These things are notorious for leaking IF the job is not done Correctly.