Thinking Thursday #1

This is the first of what I hope will be an irregular series: Thinking Thursday. My idea is to post puzzles, riddles and so on for amusement, but which will hopefully make you think a bit too. Answers will be provided on the following Sunday, so you’ll have a couple of days to mull over the questions.
OK, so lets have the first puzzle …

What says 0123, 1234 and 2345 but never 3456?

There’s no prize for correct answers other than the fun of taking part, but I would love it if you put your answer in the comments.
Answer on Sunday evening.