I thought the rule was 4 rounds?

If Hatton gets cut with an accidental headbutt and 4 rounds have been completed, then they go to the scorecards.

If Hatton gets cut with a punch and can't continue, then tough luck for Ricky, because he loses.

Floyd has almost no history of headbutts because he's usually leaning backwards. If Hatton gets cut in this fight, it will be because of a punch, and therefore the scorecard rule has no relevance.