First, within Appearance > Menus, click Screen Options at the top and ensure CSS Classes and Description are ticked.
Also tick 'Custom Links' is not already ticked.
1. For the menu item you want to turn into a mega dropdown when hovered over, we add a couple of classes to the CSS Classes field. This will make it full width and 4 columns.
menu-full-width col-4
2. Next, we nest our Columns under the main About Link like so: