STEP 1

Open your childtheme - child-theme/config/theme-supports.php and find this line

'genesis-footer-widgets'          => 3,

change to

'genesis-footer-widgets'          => 4,

STEP 2

Open your style.css file - child-theme/style.css and find this line

    	.footer-widget-area {
    		float: left;
    		margin-bottom: 0;
    		width: calc(100% / 3);
    	}

change to

    	.footer-widget-area {
    		float: left;
    		margin-bottom: 0;
    		width: calc(100% / 4);
    	}