Several options here depending on your situation.
If you have the Plus Plugin, you can use the custom widget width option in the footer widget area, to create a layout of 4 widgets, 3 on one line and one below, as explained in the guide article below
Widget Area Layouts – Weaver Xtreme Guide (weavertheme.com)
Alternatively, you can create that layout with a single Flex columns widgets placed in the footer widget area as explained below
X-Plus Flex Columns Widget: Usage and Customizations – Weaver Xtreme Guide (weavertheme.com)
If you do not have the Plus plugin, you can create the 4th area below the 3 widget, using the Footer HTML area, or eventually the Post Footer HTML Insertion Area as illustrated in the article below
Layouts and HTML Insertion Areas – Weaver Xtreme Guide (weavertheme.com)