From the Super Mario Wiki
A Slow Block is an item that appears in Mario Party. It is bought at the Mushroom Shop for 100 coins. It's the opposite of the Speed Block, since when it appears on a player's turn, the player can only roll a 1, 2, or 3.
Oddly enough, the Slow Block has a four on the top of the die even though it only allows the player to roll a one, two, or three.