Four Obsidian Slate can be used to create on block of Obsidian. Obsidian Slate is also used to craft Obsidian Tools and Armor. It cannot be used to craft Slate Tiles or Flint and Steel.
Prior to Minecraft 1.6.2, the recipe was one block of Obsidian on the crafting grid yielding two Obsidian Slate. Some early versions of MC 1.6.2 still use this recipe.In Minecraft 1.6.2 and later, the recipe has changed to two Obsidian on the crafting grid, one over the other, producing four Obsidian Slate. This was done to prevent conflicts with the Mystic Ore mod.