As it currently is when you are at stage 4 you do not heal at all, I have a problem with this mainly because Vampires are supposed to heal quickly as implied in the Lamae Bal flashback, this passive is as unlorefriendly as it gets, It does seem as if ZOS wants to promote the idea of a Vampire being someone who cannot be healed as effectively as normal as seen with the PTS changes to Blood to Blood, I believe this following idea of mine would actually improve upon their vision and make the Vampirism system feel more consistent.
At stage 4 instead of not healing it should be the exact opposite, your base health regeneration would be significantly higher based on the Vampire stage you are in, to balance this it would however reduce your healing taken from outside sources by 3%/6%/9%/12% per stage, the base health regeneration would be significant enough that a higher stage Vampire could rely on their health regeneration to save them, something like 250 extra health regeneration per stage so at stage 4 they would have +1000 Health Regeneration on top of their normal Health Regeneration.
What do you think of this idea?