Thank you for all the feedback to my last blog, both personally and through facebook.
The answer is that information is communicated via the four remaining cards. It is not obvious how it is done but take a look at the article by Colm Mulcahy, which explains all.
If you are interested in these sort of puzzles I fond this one at Gurmeet Singh’s blog. It is an excellent blog and contains many other puzzles like this.