Resizing Image Gallery for different widths

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I have a gallery with 8 images displayed on a desktop with 4 columns and 2 rows - which is fine.  

Currently the image gallery changes to 3 columns and 3 rows on a smaller device type (approx IPAD Portrait size) - this doesn't work for me as the last 2 images are on a row with a gap next to them where the 9th image would have been

and then to 2 columns and 4 rows on an even smaller device type - which is fine.

I would like to edit the css so that the images are either shown as 4 columns on a large device or 2 columns on a small device - ie I want to remove the 3 column option.

Can someone help show me what css to add 

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