If you consider a reference as a bridge to the island, you first create three bridges from main to the three islands. You then build a bridge from the island named i2 to the island named i3, from i3 to i4 and from i4 to i2. Each island now has three bridges coming in: one from main and one from one of the other islands.
If you then assign null to your references, you break down the bridges from main. The islands cannot be reached from main anymore, but they still have bridges between each other. That's what's called an island of isolation as Paul has said.