Instructions
Reach the four-player area within the Collector's Lair by grabbing (hold down the R1 button) onto the big cloth ball which is labeled "x4"; you will see the ball when you set a prisoner free after the long ascent on boxes that appear toward the end of the level.
Move one player on to the raised button in the special area to which you are taken. Move the second player onto the next button; the third onto the button that comes up after that and finally, the fourth player onto the final button that raises up.
Get the items that appear; one of these includes the Chinese Dragon Mask.