To edit the Template: 

Navigate to Templates > 404 Template > click "Edit with Elementor."

Full listing of Elementor Widgets: https://elementor.com/help/widgets/

Editable Content:

1. Border: Edit Section > Style > Border.
2. Background: Edit Section > Style > Background.
3. Inner Section
   1. Column 1:
      1. Heading Widget
   2. Column 2:
      1. Heading Widget
   3. Column 3:
      1. Heading Widget
   4. Column 4:
      1. Heading Widget
      2. Heading Widget
      3. Search Form Widget
      4. Button Widget
      5. Button Widget
      6. Button Widget

Button Note:

These buttons are set on the same like by using inline positioning. To edit this, go to Edit Button > Advanced > Positioning > Inline(auto)