You must be able to target four cards in a single graveyard in order to activate Pestilent Cauldron’s last ability.

Pestilent Cauldron’s second ability counts the total amount of life gained without considering any life you lost during that turn. For example, if you lost 2 life and gained 2 life earlier in the turn, your opponents will mill two cards.

You may cast Restorative Burst with no targets. If you do, each player will gain 4 life and you’ll exile Restorative Burst. However, if you choose one or two target cards, and each of those cards is an illegal target as Restorative Burst tries to resolve (usually because something else moved them in response), Restorative Burst won’t resolve and none of its effects will happen. No one will gain life, and Restorative Burst won’t be exiled.