An early achievements leak for Viva Piņata: Trouble in Paradise has revealed it will ship with 4-player online co-op.
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It's the 'Full House' achievement that gives the game away.
To unlock the achievement, you have to "Play 4 Player Online Co-op for 1 hour". There's not really much room for guesswork left after an achievement description like that.
Microsoft hasn't confirmed anything yet, so we just need to wait and see if the achievements are real...