t.php

<?php
function nl2br_limit($string, $num){
    return preg_replace('/\n/', '<br/>', preg_replace('/(\s{'.$num.'})\s+/','$1', $string));
}
$cadena = <<<EOF
uno dos tres










bla bla






bla bla bla bla
cuatro
EOF;

print nl2br_limit($cadena,2);