Be careful to not be out too long for a 4 year old's attention span, and have a Plan B ready if you reach this limit. Your objective should have less to do with catching fish than having him want to do it again. If he wants to bail, or watch Sponge Bob, bail - you want to avoid a "Dad wants to go fishing again Mom, make him stop!". I took my daughter first when she was 3, and now at 14 she ties her own flies, has her own waders, etc. - but the first time was with worms in a pond for blue gills, and lasted maybe an hour. Bring snacks and drinks, and have a plan for potty breaks - easier with a son, who might get a kick out of doing it over the side of the boat (more of an issue with my daughter - she didn't care, but my wife was horrified when the event was described after...). Try to combine with a side trip that is kid oriented - shirt hike, stop by a hatchery to see the fish, etc. Have fun - going fishing with your kid over the years is a blast.