Based on my own math I did a long while ago, Flamethrower is worth using on 4+ target if!
-Bio Blaster isn't coming back in the next 10 seconds
-Chain Saw isn't coming back in the next 10 seconds
-You have a low Heat gauge (0~10)
-You can't ensure a full duration of Hypercharge

If those conditions aren't filled, you're better just spamming Scattergun, especially if you a Reassemble ready for CS/Scattergun.
But remind yourself we're talking about ridiculous gain of potencies that might not even make you gain a second.