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.
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.