After you use portals to get past gaps in the floor, you reach Test Chamber 04.

Put the Cube on the Button

Nearby you see a red button. Continue into the chamber and you will see a weighted storage cube fall into a big pit. Create a blue portal on a wall in the pit, then go through the orange portal near the red button to retrieve the cube. Place it on the red button to open the chamber lock.

Enter the elevator and it will take you to Test Chamber 05.