Hey Czech,
That's a trick question. Is this four inch drop in a stock vehicle? Stock location? The answer would be stock ride height minus four inches. Then you need to take the stock wheel/tire height to new tire height difference into consideration. Stock tire height minus new tire height, divide by two, add that to the four inch drop. That should give you a close idea on ride height.
But... If it isn't a replacement in a stock location... You gotta figure it out.