There are 4 slots but I have 3 blue cubes. How do I open this door in 'The Final Hurdle'?

In Antichamber, one of the first puzzles with the blue gun involves 4 slots, but I have only seen 3 blue cubes on this floor. How do I proceed to open this door?

The grate thing right in front of you is an elevator/trampoline thing. If you jump on it and time your jumps right, you will reach the bottom.

When you do, use one of your cubes to hold it down.

Then walk into the room at the bottom. There will be a door that requires 2 cubes to open, and will have a cube behind it.

Once you get that cube and retrieve the ones you used to open the door, get back on the elevator and remove the cube that is holding it down. Then you will be back where you started with 4 cubes.