Side by side featured posts not collapsing

I’m finishing off my first Genesis-built website, using a child theme. It works well from a responsive point of view, except for my featured posts, created using the Sandbox Featured Content Widget, on the home page, which are two wide by two rows deep, making four in all.

Everything collapses except for the two ranks of featured-posts boxes, which just get thinner and thinner.

I’ve tried every CSS solution I can think of to style them appropriately but nothing has worked.

Can’t post a link because it’s still on localhost.

https://wordpress.org/plugins/genesis-featured-content-widget/