4x-Cut does indeed attack enemies that have already lost all their HP, just as Tifa's Limit Break does. They'll disappear at the end of the attack animation. This can be annoying if there are multiple enemies on the screen and you completely miss one of them, but it's no great hardship. It's also great for single targets.

Slash-All can be useful for gaining Limit Breaks (vs Battery Caps in the Rocket Town forests) and powering up Vincent's Death Penalty, but that's about where it stops.

Flash is completely useless. It's got half the normal Attack% and only inflicts Death. If the enemy is immune there's no damage dealt. You'd be better of with Slash-All and Destruct=Added Effect. At least then you'd deal some damage.