'Each unsaved wound cased by the Hellfire Sword casuses not one but D3+1 wounds'

Two questions

1. Does that mean that if I hit an infantry block I cause a D3+1 wounds to the unit even if the individual models only have 1 wound on profile, or do I only cause D3+1 wounds against individual models.

Basically I need to know if this affects a unit, or is only really good for killing multi-wound models.

And leading on from that, if it only affects single models, suppose I role 4 wounds, the enemy takes off his one model, do I get +4 to combat resolution since it caused 4 wounds, or only +1 since that's all that was on the profile.