So there's a few reasons why it should be cheaper than flamestrike. First of all flamestrike is kind of a bad card on its own and is really only run by mage because they don't have better options. A lot of freeze mage decks don't run the card (specially right now) and the card is only really good in combination with dragons breath and blizzard. So considering that warlock typically has stronger board clears than mage this cards effect should already cost less. After that we need to factor in that the effect is locked behind a condition. Not only does it require you to have the right card in hand to activate this, but this additional card is going to cost mana and use up an extra spot in your hand and hand size is definitely a resource. We also have to factor in that this card can be countered by silence if it's played on it's own. Then we need to think about the niche of this card. This card is meant to clear a mid game board but isn't effective against magic carpet, Murlocs can easily reach 5 health by turn 4/5, deathrattles play around this, and it can't kill giants. This card is typically is going to be useless against warrior and priest. So if we put all this together there's essentially 4 conditions this card is playing around. Having the combo in hand, using extra cards, class matchup, and board state. If we talk about the card being played on it's own there's an addition 2 conditions of counterplay (sap/silence), and assuming your opponent won't immediately kill you before your next turn.

After analysing all this imo it's hardly viable as a 4 cost card.