Setting the Health card upgrade boost to +2 health/ level is fair for the zombies to deal with because it gives them a fair chance to kill humans.
Increasing it to +4 health/ level would be broken such that it would be slightly harder for zombies to kill humans when they have +20 health from cards alone at level 5, not to mention that some players are around levels 8-10 for Health, increasing their maximum health up to 120, and traits like Fit and Tough exist to increase health by 35-40 points combined alone.
There's a reason the Health card upgrade was nerfed from 3 to 2 health points/ level, and that's because zombies couldn't kill humans easily; even when they get themselves caught out, they still don't die because the extra health saves them, which is frustrating for zombie players.
I don't see the real reason you want this change aside from making playing zombie more frustrating when it comes to killing humans.