4 columns to 2 columns with Twitter Bootstrap

Answer 1

Adding to Emilie's answer you also need to reset the widths of the columns,

@media (min-width: 768px) and (max-width: 979px) {
    .row-fluid > [class*=span]:nth-child(2n+1) {
        clear: both;
        margin-left: 0;
    }

    .row-fluid .span3 {
        width: 48.61878453038674%;
    }
}