Originally Posted by
Engine
If you're going out with a 4-year-old, whatever you do keep it fun. If it turns into a misery for him, it will take an act of congress to get him on the trail next time. Low mileage, frequent breaks, fun games in camp, tasty food, etc...
And has been stated, be prepared to carry his pack as well as your own half way through the day. At age 6 my son would be dragging bottom after 4 miles or so and he would look absolutely spent at the end of 7 miles. But, 10 minutes after we stopped hiking and started setting up camp we would look around and he was off in the woods playing and having the time of his life. Their energy levels are directly related to the "Fun quotient".
Take lots of photos, you'll miss these times terribly in 25 years!