A man has four keys, but only one opens the door. Strangely, he does not try any of the keys. So how does he get inside?
The trick is that the riddle never says the door is locked. The mention of four keys encourages you to assume he must choose the correct one, even though he may not need a key at all.
Answer: The door is already unlocked, so he simply opens it and walks inside.
This is a classic assumption puzzle. The keys are there to distract you from the simplest possibility. When a riddle gives you extra information, it is often worth asking whether every detail is actually necessary to solve it.
