The interesting bit is to come up with a strategy to make 5 adaptive queries that will guarantee you know all the 4 cards no matter what the response you got to the queries.
As a simple example: Suppose I do the following (static) set of 5 questions:
- Is the first card Red?
- Is the first card King?
- Is the second card Red?
- Is the second card King?
- Is the third card Red?
You can see that there might be a situation where these 5 questions are not enough to uniquely determine the arrangement (for instance, in case you got answers Yes, Yes, Yes, No, No respectively).
More succinctly, 5 is the information theoretic lower bound but that does not mean it is achievable and that is precisely what the post is about. Is the information theoretic lower bound achievable?