Total Theme: Megamenu with column stretching full width below other columns

I’ve got a megamenu on my site which has got three columns and is working fine. My client is asking if I can make another column underneath the three, which spans multiple columns as they want to put a large banner-style link in there. Is this possible?

So what you do is you keep the Megamenu as 3 columns but add a 4th group of items. Then with this CSS you can target the last one to make it 100%:

.main-navigation .megamenu > .sub-menu > .menu-item:nth-child(4) {
    grid-column: 1 / -1;
}