I think pork loins are among my favorite things to smoke, and I do them fairly often. A couple of suggestions: a: Loins are fairly lean, so brining them will help keep them moist. Use the search function for DivotMaker's Brine for Pork Loin, it's terrific. b: the FDA now says that pork is safe to eat at 145 degrees F, and the higher temp you take the meat to, the dryer it will become. I take it out at 145, then reverse sear, and it is done to my liking.
A 4 - 5 lb loin usually runs me about 3 - 3.5 hours, although I have had them done in as few as 2 or as many as 5 hours. Get a good remote thermometer and cook by temperature, not time.
Put a small pan of water (or other liquid), disposable aluminum mini-loaf pans are great, in the smoker alongside the wood box to add moisture in the box and help prevent the meat from drying out.
Good luck, and Happy Feasting!