We 4 stack for fireballs and even with lowest HP in the party as WHM it very rarely puts me below 1k HP after it hits. SCH will usually succor before and I'll Medica II after.

We've not really tried 3 stack much but there were occasions where 3 stack has hit me for almost 5k despite the other 2 people only being hit for around 3.5 ish. No idea why, a crit maybe? But that's one reason we don't do it that way, and we have the dps to take down 4-stack conflags anyway.