Planar Collapse

• WWizards of the Coast2004-10-04
  Official Ruling
  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.
• WWizards of the Coast2004-10-04
  Official Ruling
  The ability is not optional.
• WWizards of the Coast2010-06-15
  Official Ruling
  If this card is not on the battlefield when the ability resolves, then you can’t sacrifice it. However, you still destroy all creatures.

Rulings via Scryfall