Hi there,
Thanks so much for using Gridbox.
The theme uses a responsive layout and the number of columns automatically adjusts with the screen size. Mobile phones show one column, tablets two, large screens four and so on.
That means you have to rewrite the media queries to add more columns for certain screen sizes, so this is not a simple change. You have to look into the style.css of the theme and check out how the column system works, then you can expand it.
Afterwards you can override the default CSS of the theme with a child theme or the Additional CSS field in the Customizer. An experienced web developer should be able to do that in 1-2 hours.
Hope that helps.
Best,
Thomas