Use a 4-pixel grid for the largest resolution

I start at iPad 3 resolution and scale down by 50% twice to get the three resolutions I need for iOS development of Universal apps. I would LOVE it if my highest resolution document could be set to automatically respect a 4-pixel grid when creating the PNG files. This would ensure that each lower resolution is *exactly* half the size. 

Currently I need to create a "#" rectangle shape layer for almost every graphic I create, unless the shape is already a rectangle that I know is snapped to my 4-pixel grid. So right now I create my artwork, then use "Create slice from layer" to get the full transparent area, then I draw a rectangle that pads that out to the 4-pixel grid on any side that needs it. This is the only way to make sure the pixel sizes are precise when the graphic gets scaled down