Question for the experts out there... Is it worth brining if I only have about 4 hours to do it? I was going to do sea salt, pepper, water (obviously), rosemary... Also, is there a formula for cooking on the grill for time per pound? I have one that's about 4 lbs.
You can increase the salt to speed up the brine time but there are risks. Brining works through osmosis. At first the moisture leaves the meat because the salinity is higher in the brine. Eventually enough moisture leaves that the salinity is higher in the meat and water goes back into the meat. If you don't wait long enough for the latter to occur, you end up with drier meat. In regards to cooking on the grill, it depend on the grill temp. A 4 pound roast should take about 1-1.5 hours. However you should use a thermometer to make sure because you don't want to overcook. 145 degrees internal temp should give a good juicy roast. Make sure you put the heat on one side of the grill and the loin on the other side. You don't want flare ups and uneven cooking.
Skip the pork roast and pickup a pork tenderloin instead, it is well worth the next dollar or two a pound. I usually cook basted in olive oil with crushed garlic and various spices like this recipe. Super-simple yet so freaking delicious.