Kuldotha Flamefiend's ability may target up to four creatures and/or players. Each must be chosen to receive at least 1 damage.

You choose the target(s) and how the damage will be divided when you put the triggered ability on the stack. You don't choose whether to sacrifice an artifact or which artifact you're sacrificing until the ability resolves. Once you decide to sacrifice an artifact (or not), it's too late for players to respond.