#wpbody #wpbody-content #dashboard-widgets .postbox-container { width: 100% }
@media ( min-width:768px ) {
    #wpbody #wpbody-content #dashboard-widgets:not(.columns-1) .postbox-container { width: 50% }
}
@media ( min-width:1200px ) {
    .columns-prefs .columns-prefs-3, .columns-prefs .columns-prefs-4 { display: inline-block }
    #wpbody #wpbody-content #dashboard-widgets.columns-3 .postbox-container { width: 33.3% }
    #wpbody #wpbody-content #dashboard-widgets.columns-4 .postbox-container { width: 25% }
}