Planar Collapse

Oracle Text

At the beginning of your upkeep, if there are four or more creatures on the battlefield, sacrifice Planar Collapse and destroy all creatures. They can't be regenerated.

Card Rulings

10/4/2004 This ability does not trigger at all if there are not 4 or more creatures on the battlefield. It also checks this at the start of resolution and does nothing if this is not still true.
10/4/2004 The ability is not optional.
6/15/2010 If this card is not on the battlefield when the ability resolves, then you can’t sacrifice it. However, you still destroy all creatures.