The assistant who reveals four cards to name the fifth
A magician and an assistant agree on a plan beforehand. On stage, a volunteer picks any five cards from a standard 52-card deck and hands them to the assistant (the magician sees nothing). The assistant places four of the cards face up in a row, choosing which four and in what left-to-right order, and keeps the fifth hidden. The magician walks in, looks only at the four cards and their order, and instantly names the hidden card.
There is no marking, no signaling, no talking. Everything must travel through which card is hidden and the arrangement of the other four.
How can they always succeed?
Your answer
This one is open-ended. Work it through, then check your reasoning against the full solution.