Product display – products per line

Hi,

I am having trouble with the way products are displaying on my shop page. Previously there were 4 products shown per line on laptops and tablets, but now only 2 are displaying on some devices. I assume this is something to do with the screen resolution. How do I adjust the settings so that 4 product photos display on smaller screens?

Thanks
Nicki

The page I need help with: [log in to see the link]