4 columns to 2 columns with Twitter Bootstrap

Answer 1

I found out that Zurb Foundation (http://foundation.zurb.com/docs/grid.php#threeBlockEx) offers this fonctionality, but I want to continue with Twitter Bootstrap. So I've managed to add a custom rule, based on the technique that Foundation uses :

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