I would not say there is a set age when a child can handle these issues, but 4 might be a little young for most. Don't assume he is ready for anything. Be in a constant state of evaluation of him, and be willing to have him wait. (It doesn't sound like this is true with you, but sometimes we parents want something for our kids that they are not ready for, and we're doing it for ourselves even more than we're doing it for them.)
One of the things that most good dads have going for them is that Junior admires Dad and wants to be like him, so there is a natural affinity that you can build on. Make all your trips to the woods interesting, and not too long, and explain everything you can. Teach respect for the land and the animals it supports. At that age, I'd suggest target practice, but not to use 3D targets. Remind constantly of the danger of the weapons. Teach him the rules, and "test" him by asking him things like, "Do you remember the rule about where to point your gun?" Affirm every correct response, and use wrong answers as a teaching opportunity. Affirm him every time he demonstrates that he has learned, and correct him gently when he shows evidence that he has not learned.
We have to remember that we live in a different day than when most of us came up. Video games, one thing I wasn't exposed to as a kid, seem to me to convey some wrong ideas about life and death. Maybe that's why I don't much like video games about hunting. Plenty that is on television conveys a lack of respect for life. Maybe that's why I don't much like watching crime shows. Hunting is viewed by many today as a violent sport. Do not allow him to view hunting that way.
I'd say begin to involve him in the things you do, but don't overwhelm him. Intentionally set the stage so that he yearns for hunting, and begins to see it as a privilege and an honor to go with you. Don't let him take it for granted.
I'm sure others have some good ideas worth hearing. This should be a good thread.
When the Everyday Hunter
isn't hunting, he's thinking about hunting, talking about hunting, dreaming about hunting, writing about hunting, or wishing he were hunting.