Oracle Text

When Hecatomb enters the battlefield, sacrifice Hecatomb unless you sacrifice four creatures.

Tap an untapped Swamp you control: Hecatomb deals 1 damage to any target.

Card Rulings

10/4/2004 If you have less than 4 creatures, you do not partially sacrifice. You pay 4 creatures or none.
10/4/2004 The sacrifice is done as a triggered ability just after it enters the battlefield. It is not done on casting.
10/4/2004 You choose whether to sacrifice creatures or not on resolution. If not, then you sacrifice Hecatomb. You can choose to not sacrifice creatures if you no longer control this card on resolution.