Oracle Text

At the beginning of each opponent's upkeep, Skullcage deals 2 damage to that player unless they have exactly three or exactly four cards in hand.

Card Rulings

12/1/2004 Skullcage’s ability deals damage if the player has zero, one, two, five, or more than five cards in hand when its ability resolves.
12/1/2004 Skullcage doesn’t check hand size when it triggers, only when it resolves.