Reply To: Added additional widget areas, now responsiveness is broken

and you’ll also need to add some css for the 4th welcome widget area since it will show up off and to the right. So doing this will fix it for the responsive area –

.welcome-feature-4 {
    float: left;
    width: 100%;
}

The other welcome widgets float left and are at 100%, so that’s why you would need to do it for this one as well.