If you add additional up-sell products to your product, they are displayed in a table, at the bottom of your product’s view page. By Default they are set 4 in a row, but this can be changed.
Locate the file app/design/frontend/your_package_your_theme/layout/catalog.xml, or copy it from the tefault package if it doesn’t exist under your package layout folder yet.
In this file look for the block that starts with
Change the line
<action method="setColumnCount"><columns>3</columns></action> to read
<action method="setColumnCount"><columns>4</columns></action> (when 4 is your desired total in a row), and you’re good to go!
If you want to make further changes to the block’s HTML – the template file is located here: app/design/frontend/your_package_your_theme/template/catalog/product/list/upsell.phtml.