Life Drain is a state like Stunned or Knocked Down. If a unit has a damage line that reads 4+ Life Drain 1, then an opposing unit would take 4 damage (basic damage) plus 1 point of Life Drain damage(the applied state). In the same way that a damage line of 8+ Knocked Down will cause 8 points of damage and the knocked down effect. That is how we have always interpreted the rule.
I think part of the confusion is that Foulques card only has -
1 Life Drain 2
Where as other models have -
4 4 + Life Drain 1
Right. So we need to identify Life Drain (x) is a damaging affect. If so, a model with the stat line listed above (hits: damage: 4+life drain 1) a model will take 5 points of damage or does it only take 4pts of damage.
Some confusion on the cards then.
From the rulebook, PG 73:
Life Drain X
Damage of this type ignores PR and Immunity
(other than Immunity to Life Drain). The unit that
causes this type of damage will recover a number
of LP equal to the number of LP that the unit suffering
this type of damage actually lost.
This is DAMAGE. So a model with Life Drain does damage even if the power table says Damage: 1 + Life Drain 3, you will do 4 damage, and 3 of it will come from Life Drain.
Hope that clarifies!
EDIT: New post above mine clarifies better as it posted as I was posting my post.