4 is a great age to get a kid started. And your instincts are good, you want to start letting him catch something at this age.
I didn't know the Eno, so I searched for it and see that it has a lot of swift, moving water. Fish that live in current can be difficult to catch for kids.
I'd reccomend you find a local park lake or pond and take him there instead.
Find a local tackle shop and take his little rod in there and tell them to rig it up for your son to catch
bluegill. ( A small, agressive, hand size panfish). They should tie a small hook on, with a cork (float) a couple of feet above it along with a light lead BB split shot weight about 4" above the hook to get the bait down.
For bait, buy a box of red worms or get a bucket of crickets. The bait alone will entertain him for the first few trips.
If you can't find a tackle shop, a Wal-mart will have what you need all except live bait. There is a packaged bait Wal-mart will have that's called Berkley "Gulp!" It's a bio-degradeable substitute for live bait and you can cut it up in little pieces to put on a small hook and it will work fine for bluegill. It won't matter what kind or color of Gulp! you buy, just something you can cut into little pieces.
Here's the deal. After you get the rigging right, go to the grocery store and buy some canned fish flavored cat food and a loaf of wheat bread. Mix it all together in a tupperwear container and take it with you. When you get to your fishing spot, sprinkle that "chum" in the area and put the bait out there. The cat food/bread mix will attract the bluegill to you, and it should be entertaining watching your cork go down.
If you chum the water and can't get a bite after a little while, you will need to move to another spot, they aren't there.
Good luck!