If you have trouble with killing a conflag in time doing a 4-stack, you likely don't have enough dps to kill twintania before the enrage. Or simply, your dps needs to be better about how they burst the conflags.

Having said that, you likely do not have enough HP for a 3-stack. You would want everyone closer to the 4400 of your whm. Fireballs in a three-stack can sometimes do more than ~4.6k. While those are rare, it would leave you in a position of being at the mercy of RNG.