Here is what the walkthrough says:
Stone pedestals: Go to the heart of the chamber. Look at the 4 stone pedestals. Review the Vatican documents.
The sequence of the planets in conjunction is: Saturn, Jupiter, Venus and Mercury.
Start with the pedestal at bottom left while facing the portal: bottom left - Saturn, then do bottom right - Jupiter, then top right - Venus and last top left - Mercury. Sylvie will make a remark if it is correct.
Place the cube on top of the pedestal
I also found this walkthrough with pictures.
Scroll to the end of the walkthrough and you will see the information about the stone pedestals.