In the template /customer/main/products.tpl change the line
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")}
to
Code:
{if $usertype eq "C" and $config.Appearance.products_per_row ne "" and $config.Appearance.products_per_row gt 0 and $config.Appearance.products_per_row lt 5 and ($featured eq "Y" or $config.Appearance.featured_only_multicolumn eq "N")}