From what you wrote I think you may be confusing cropping and resizing.

I use an Olympus E-20, 5MP. All my images are 3560 pixels (35.556 in) X 1920 pixels (26.667 in) at 72dpi.

When I crop in Photoshop (for printing) I set my dpi to 300. Keep in mind that cropping and resizing are two different things. Likewise if you fix your marquee to 4x4 it is still different than cropping.

If you have four images coming off your camera, and they are the same size as I mentioned above, and you crop each one to 4x4 inches at 300 dpi they should all be the same size.

When you create your layout the background NEEDS to be the same dpi as the photos you are going to be putting into the layout. In this case 300dpi.

Hope this helps.