Hi George,
So upon reflection, and based on your reply, Poster mode is not the way to approach this.
Correct. Poster mode is designed to print an image larger than the paper size. Although it could be used for your requirement, it's very fiddly and you don't get a proper preview of what it will end up like.
There are 2 other ways to do what you want. The first is more or less as you describe using print cropping.
For your square images I would crop the image square (crop ratio 1:1) in the QU image editor first.
Then set a print size of 6x6" and add 4 copies of the image to the queue with crop scissors on.
In the page editor, zoom to 2.0x and position the crop as required for each image. The latter has to be done carefully and using the High Precision Crop tool will help
See my first screen shot below of the page preview. I used Compact placement, added a small white + border to separate the images and guide lines.
The other way to do it is to make 4 copies of the image and in the image editor, set a 1:1 crop ratio and the position the crop in each image as you want. QU gives a pixel size read-out as you crop which will help you to get the correct size and be consistent.
See second screen shot. (NB. it does not show the full editor screen).
Have fun!
Terry