I have a 4 lb pork loin to roast, approx 2 hours. I also need to travel 1 hour distance. The pork is the main entree. Dinner is set for 6 pm. What is the best method to cook the roast so that it remains moist? I cannot arrive much before 5:45 and we can eat a little later. Cook for 1 hr, travel for 1 hr and complete 1 hr cooking at destination? Any combo of these?
That's actually just about perfect, since meat should rest before you slice it. Start the roast at 3. Cook it for 2 hours. Wrap it in aluminum foil. Put it in something to retain heat (a cooler, or even just a box with some towels). Drive 1 hour to your destination.
The muscle fibers will re-absorb the moisture, making it perfectly juicy. Without resting, it will seem "juicy" when the juices pool on your slicing board, but that's moisture that's lost. The sensation of juiciness is strongest when it has re-absorbed the moisture among the fibers, so it comes out in your mouth when you chew.
Wrapping it in aluminum foil will keep it warm as you go. Heat will equalize throughout the roast. Since it cooks from the outside in, the outermost layers are hottest. Take it just to the very bottom of the cooking range (145°F), or even a little lower, and let the carryover heat from the hotter outer layers finish the cooking. It will arrive perfect for you to slice and eat.
If you're eating any later than, say 6:30, then keep it warm in the oven at 200 or even 180 if your oven will do that. Another hour of warmth will still be perfectly safe, and won't go too far towards moisture loss.
This sounds like a good application for sous vide cooking.
If you haven't heard of it, the idea is that you seal your food and submerge it in water that is held at the temperature you want your food cooked to. The advantages are food that is cooked very slowly and can't overcook. Professional cooks use this to be able to prep their food in advance and still have it come out perfectly when it is served.
J. Kenji Lopez-Alt has a good description of how to accomplish this effect with a cooler. The cooler will keep the water at the target temp for a long time. http://www.seriouseats.com/2010/04/cook-your-meat-in-a-beer-cooler-the-worlds-best-sous-vide-hack.html
When you get there, 15 minutes will be enough time to just brown the surface under a broiler and serve.
If you aren't feeling so adventurous and want to use a traditional method, you should roast the pork completely, wrap it in foil, and pack it in a towel and then a cooler. It will retain most of its heat for an hour. You might need to warm it up a little but that won't dry it out.