I'll make a thread about this...
6 Fire IV is still a dps gain.
The issue is that if you move at all without Sharpcast up to get the F3 proc, you lose a Fire IV (due to AF III leniency).

5 Fire IV + the necessary B4 is a pps loss.
The guy is probably predicting a mechanic for which he cannot do it and he bypasses it half the time (not always though, if you see carefully).
From top of my head, up until gaols+puddles I can make it work.

But for churning+knockback, for levinbolt+cloud+knockback and sometimes for puddlesx2+churning, I just do not have enough casts and bypass B4 myself.