as with the others, good for you taking your 4 year old fishing.
My son, Daniel, is now 15 and is still my fishing buddy (and has developed into quite the fine fisherman I must add as he regularly outfishes me).
With a child that young, I think I'd focus on turning over rocks, playing in the stream and enjoying a nice day out in the woods. If there is any kind of moving water at all, things can happen so quick that even a momentary lapse in focus could result in tragedy. If you're landing a fish, looking up the stream to the next run or fiddling with your fly line as you're trying to dislodge a fly from a tree, you could look up and see something you really don't want to see.
Take your son or daughter fishing, yes, absolutely it will be time you will remember for the rest of your life, and when he or she is older, it will be times you can share and both of you will remember the rest of your life. 40+ years later, I still have very fond memories of fishing with my grandfather.
I just think most of the streams in the Smokys wouldn't be my first choice.
Metcalf Bottoms may be a possibility if the water is still low (nice picnic area, plenty of area to splash around, plenty of other people) and it will have some fish to cast to. fishing would be great, catching may be a bit slim.
Hope you all have a great time.
By the way, what part of Kentucky are you originally from?