Cards puzzle

I give you 4 cards from a deck of 52 playing cards. You intelligently select 4 out of those 5 and give those to your friend. He analyzes the 4 cards and tells you the fifth one. You have to pre-decide a strategy to do so. The puzzle is – What will the strategy be?

On a higher level - I shuffle 4 decks of cards and then select 6 cards from the shuffled decks. This time you have to do the same with 5 cards. What will the strategy be ?