Magento — Change Category Product Grid Layout from 3 Columns to 4 Columns #M1
https://collaboration133.com/magento-change-category-product-grid-layout-from-3-columns-to-4-columns/2384/
<action method="setColumnCount"><columns>4</columns></action>
<action method="addColumnCountLayoutDepend"><layout>empty</layout><count>6</count></action>