Haste is only every one additional attack.
Really, I don't think it is worth worrying about the maximum potential of stacked-up d4 because it takes too specific a build (monk + fighter + this weapon + haste from somewhere + action surge), and is limited in application because you have to get lucky and actually score nothing but hits on that rare instance that all limited resources are spent at once (ki + action surge + spell slot/other magic item).
And maximum potential, you aren't actually going to see that much more damage than something like a full fighter with a flame tongue greatsword (roughly 20% more at absolute best conditions if my math was correct).