I'm going to go against the grain here, and say too young.
At four, he is not going to remember much in 6 years time. He will have a limited attention span and energy level for big walks.
By all means take him, but involve him in the other side of hunting, the things that are arguably more important than the shooting part. Things like picking up decoys, plucking ducks, looking for possum eyes in the light, carrying the rabbit in his special "hunting' pack. Teach him about the animals you hunt, the weather etc. Equip him with a usable skillset so when the time comes for him to shoot he will be keen as mustard, and the actual act of learning to shoot at the age of 7-8 is a formality because he has been technically 'hunting' since he could walk.
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