Michael on "[Theme: Pictorico] I want to add Full-width fourth footer widget"

the below should work when you are using all the other three footer widget areas and the fourth footer area.
'Footer Sidebar 4' should then be full width.

/* =Widgets 3 + 1 */
#secondary.widget-areas .widget-area:nth-child(1),
#secondary.widget-areas .widget-area:nth-child(2),
#secondary.widget-areas .widget-area:nth-child(3) { margin-right: 5%; width: 30%;
}
#secondary.widget-areas .widget-area:nth-child(3) { margin-right: 0;
}
@media screen and (max-width: 885px) { #secondary.widget-areas .widget-area:nth-child(1), #secondary.widget-areas .widget-area:nth-child(2), #secondary.widget-areas .widget-area:nth-child(3) { clear: both; width: 100%; margin-right: 0; }
}
#secondary.widget-areas .widget-area:nth-child(4) { width: 100%; clear: left;
}

add it to style.css of a child theme or via a 'custom CSS' plugin.

due to the programming structure of the theme, this will not work if you do not use all the 'Footer Sidebar 1' and 'Footer Sidebar 2' and 'Footer Sidebar 3' areas.

roughly tested in various browser sizes in Firefox, but not tested in any actual small devices like mobile phones, etc ...