How long to bake a 3.78 pound frozen pork loin roast?

From personal experience, the best way to bake a frozen pork roast is to seal it well in tin foil with a bit of added water and bake at 200 to 250 degrees F for up to 4 hours or more. Just wrap it, put in the oven and leave it for 4 hours. After 4 hours, check it for doneness. You are looking for juiciness and tenderness. The meat cooks slowly in its own juices and becomes fall-apart tender. If it isn't as tender as you'd like, as the defrosting will require a while, re-wrap and bake it longer.