It's okay if the chicken is a few ounces more or less than 4 pounds. You may just want to check the internal temperature sooner or later, depending on the size of your chicken.
I like to use a mini food processor to chop the sage and garlic so that it's super fine before mixing them with the salt, pepper and oil.
You can use another ovenproof pan for this meal if you don’t have a cast iron skillet. Just lightly oil the bottom of whatever pan you use.