We were actually in the middle of a totally different experiment, when the bubbles in the bottle caught my son's interest. He kept flipping the bottle one way and then the other. Finally he asked, "Why do the bubbles go to the top?"
It was the perfect moment to talk about force. I asked my own question, "Why does the water go down?" It's because gravity pulls on it. Gravity is the name of a force. What are the bubbles? Why does gravity pull on the water more than air?
Now we had moved into density. Do you remember when you were crammed in the tent with all the pillows and your sister? That was more dense. When you were alone in the tent, that was less dense, and gravity pulls on more dense things more.
If his natural interest is any clue, four-years-old is the perfect time for this little boy to master physics.