Flex 3.5 / Flex 4 Strange Scaling/Positioning problem

Question

I have a simple block of code to demonstrate an issue:

package
{
import flash.display.Sprite;
import flash.events.Event;

[SWF(width="400", height="400", frameRate="24", backgroundColor="#FFFFFF")]
public class sandbox extends Sprite
{
    public function sandbox()
    {
        graphics.beginFill(0xFF0000);        
        graphics.drawRect(0,0,stage.stageWidth,stage.stageHeight);
    }
}
}

Now if i set the complier to flex4 and open the swf file in the browser(so that it takes the whole screen) it works fine, how i would like it (a 400x400 box in the middle), but if i compile in flex3.5 the stage gets shifted to the right, and scaled up (cannot see this in above demo, but it does).

What is the reason for this? i need to switch to flex3.5 for flash player 9 compatability.

Thanks in advance.