This is interesting, but debatable whether it is doable.
I wonder whether having it give you +1 armor in addition to 4-5 hit points would be worthwhile.
that would be extremely OP.
Me wallace and linden were having a conversation on IF yesterday (seeing as we have high IF), and making IF give +3 HP would be perfect imo. The thought of adding armor is insanely OP, Imagine this, a big tin can pumping IF, and making it +1 armor +2 HP. Imagine the full plate with 10 Armor higher? Ridiculously bad idea. +3 HP would be good.
But at +4 Hp.... +40 HP for 10 IF? That's double that of a normal non-IF-getting-character alone. You know, I'd say it would definitely make IF something a build would center around, but I think it would be... not necessarily a bad idea -- but terrifying to people abusing agility stacking to outspam opponents. It would be interesting.
I vote for +3 HP for IF from an objective standpoint.
I vote for +4 HP for IF from an experimental & subjective standpoint.