You can create a layout to do that.  Set your print placement to Freehand and add your 5x7 background to the page.  Then scroll to the bottom of the thumbnails and add a 4x6 of the "Template" thumbnail and drag that on top of your 5x7 background image.  Once you've done that and have things aligned the way you want, you can drop down the image placement and select "Save Current as Custom Layout" and give it a name.

Then whenever you open that layout (via that same print placement dropdown under the live view) and select "Load Layout" and choose that one, it'll be your 5x7 background with a 4x6 placeholder on top.  As you add thumbnails to print (via the + button on thumbnails), they fill the red templates/placeholders on top of the 5x7 background image.

Regards,
Mike