As Meat Hunter said, when it comes to times, the answer is a varied as there are members and smokers. I know this is frustrating when you are starting out, so give yourself plenty of time. The 225 - 250 is a pretty standard temp for most of the items being shown off on the forum. Why? Because when you go low and slow, the reward is in the taste. Yes, any piece of meat can be cooked faster, but it can't even be compared taste wise.
Give yourself at least six hours for this first loin of yours. If it hits 160, wrap it up, put it in a cooler, and relax. If it stalls, you won't be having an anxiety attack! You should have no problem smoking a 4lb loin in six hours at 225 - 250.
Hang in there, it will turn out great.