The 1/4 inch barbed hose connectors don't have threads. They have 2 or 3 ridges (they call the barbs), these ridges/barbs are there to keep the hose and connectors from coming apart while in use. Their not for attaching them to the 4 inch pipe. You can make holes large enough in the 4 inch tubing for the barbed connectors and 1/4 inch tubing to go through, then connect the 1/4 inch tubing inside the 4 inch tubing. But that will just make everything much more difficult.
Really all you need to do is run your 1/4 inch water line along the 4 inch tubing. Attaching it securely with either small plastic clips (made for the 1/4 inch tubing), and screwing the clips into the 4 inch tubing. Or by snugging down the 1/4 inch tubing by wrapping some zip ties around it and the 4 inch tubing. Once you have the the 1/4 inch water line running the length of the 4 inch tubing. Cut the line in-between the plants and insert a barbed "T" connector. Then run a small piece of the 1/4 water line up to another "T" connector, then a short line going to each plant on both sides of that connector.
Or you could just use one "T" connector and one line running from the horizontal 1/4 inch water line going to each plant. Either way each plant has a line going to it. From there you can either use dippers (they might clog), leave the line open, and adjust water flow to low. Or cap the end and poke holes in the 1/4 inch line over the pots so water drips out, soaking the
growing medium.
Another option is once you have your plants in place, run the 1/4 inch water line along the top of the 4 inch tube, and over the top of the baskets. Again attaching it to the 4 inch tubing by clips and screws, or zip ties. Then just poke holes in the line above the baskets, so water drips out soaking the growing medium. That way you eliminate all the "T" connectors.
There is no one way to do it, just what works best for you.