This is about the latest version (v3) of the mystic class from Unearthed Arcana.
Order of the Immortal mystics get the Immortal Durability feature at 1st level when they choose the subclass, part of which says:
Starting at 1st level, your hit point maximum increases by 1 per mystic level.
At 4th level, do I gain 4 total HP, or 1+2+3+4 for a total increase of 10 HP?
The Immortal Durability feature is comparable to the Draconic Resilience feature of a Draconic Bloodline Sorcerer, in that both raise your HP maximum and AC. The language for Draconic Resilience is clearer, possibly because it's officially published material:
Draconic Resilience: As magic flows through your body, it causes physical traits of your dragon ancestors to emerge. At 1st level, your hit point maximum increases by 1 and increases by 1 again whenever you gain a level in this class.
I believe the language in Immortal Durability ("your hit point maximum increases by 1 per mystic level.") is meant to be shorthand for the language used in Draconic Resilience ("your hit point maximum increases by 1 and increases by 1 again whenever you gain a level in this class."). Presumably, they will use the more clear wording if the Mystic class is ever published.
This certainly makes most sense from a balance perspective, because if you continue to use the other method (1 + 2 + 3 + 4...), the feature will add a whopping 210 HP to your max by level 20!
