At 4th Level, you gain +4 total 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:
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!