Because of the diffculties in sellotaping up the inside of a tube, three, not four, strips were used up each tube, so that one 'seam' in each tube was taped on only one side. The drawing shows two tubes with 'seams' taped on both sides. The tubes were then taped together, joining sellotape shown in green. At the leading edge at each connection point between two tubes, I put a small piece of sellotape round the front joining the top panels together from the point where they meet the lower panel up just the width of my sellotape. If you are making tubes with sellotape, you will find taping the inside of the tube difficult when closing.
Some tapes just sticks too well to plastic to control it inside the tube.
The simple solution of that fourth piece of tape, is to pull the tub inside out, and then tape it. Also a double sided tape sandwiched between the two top pieces would make the joint virtually indestructible, and stop any sand or grit getting into the length of the joint. I followed your suggestions to finish my panflute. The kite is purple, orange and green with a 50' orange to purple graded tail. After I sewed these openings shut I still had problems getting it to fly.