How to make homemade airlock?

You made a blow-off valve. Then, you discovered the weakness of it. The next time you make wine, leave more head-space. In the meantime, you could try replacing the stopper and pipe with a condom. Using a commercial airlock (only a dollar or two) would not have prevented your problem. Four inches would probably be fine in a plastic bucket fermenter but a demijohn has a narrow neck, so there isn't much room for expansion.