You can...
A) insert a “separator” a transparent element between the 3rd and 4th that will clear: both, span a height of 1 pixel, occupy the entire width, and then margin-top: -1px at 4, 5, 6, so there’s no vertical spacing of 1px.
B) use inline-block instead of float, for example: http://jsfiddle.net/gLcNm/16/
, div- , css-hack IE, inline .
C), div- "" div:
<div class="row">
<box><box><box>
</div>
row, .