Check to see if the meat has been enhanced (injected with a saline solution). If so, cook as above. if not, I would brine it to enhance the moisture. I use 1/2 cup Morton Kosher Salt, 1/2 cup brown sugar, 1/4 cup soy saue and some of whatever rub I use on the loin mixed in a gallon of water. You don't really need a gallon for a 4 lb. loin so cut it all in half. Brine for an hour per pound, rinse and dry and let it equalize in the fridge for another hour per pound.
for rub, salt, pepper, granulated garlic and granulated onion are good. Some herbs, like a poultry seasoning mix, are also a nice addition. For store bought rubs I use Plowboy BBQ's Yardbird a lot.
BTW, I cook loins at 275 until they hit 135-140 internal, rest and slice.