I know this isn't directly answering your question but what about...
In admin > General Setting > Appearance Options...
admin/configuration.php?option=Appearance
Change "Display products list in multiple columns (1-3) (leave empty to use standard format):", to 4
Then in products.tpl locate...
Code:
{if $usertype eq "C" and $config.Appearance.products_per_row ne "" and $config.Appearance.products_per_row gt 0 and $config.Appearance.products_per_row lt 4 and ($featured eq "Y" or $config.Appearance.featured_only_multicolumn eq "N")}
and change the part $config.Appearance.products_per_row lt 4 to...
$config.Appearance.products_per_row lt 5
Hope this helps.