Fullwidth portolio – columns settings

Hi
I use a fullwidth portfolio to this play products on this page: http://diyjewellery1.wpengine.com/3d-designs/ (hosted on WPengine) (see private content to access website in development).
I displays 4 columns of poducts fine on larger screen but when I view the same page on my small 15″ laptop, I only see 3 columns of products. I expected to see 4 columns and the images adjust to the small width. It looks like the 4th products would almost fit.
How can I get 4 cols on the smaller screen size, then get 3 cols perhaps on my ipad.
I thought this would be more responsive and automatically adjust accordingly.
Please help me find a solution for this
Thanks
Lyse