Just a clarification. If a creature (in this case a wererat) has DR10/Silver, someone with a magic weapon that has a +4 bonus still will have 10 HP reduced from their damage. Is this correct?
It's a bit obscure, but a +3 or higher magic weapon actually counts as cold iron/silver for overcoming DR. There's a table in the glossary page here that lists the different DRs overcome by different levels of magic weapons.