Once you have your four crystals, you need to transport them
in the portal. Place crystals 1 and 3 (numbered in the order
that they appear), pass them, and put crystal 1 in the portal.
Go back with crystal 3 and switch it with crystal 2 and 4. 
Pass them and put them in the portal. Take crystal 1 and go 
back. Take crystals 1 and 3 in the portal. Another crystal 
will appear there. Click on crystals 1 and 3 and you are done.