When a ship is upgrade from tier 3 to tier 4 in starfinder it says there is a
hp increase. You can see this in the starfinder core rulebook, page 294, table 9-1. It is also on table 9-1 here https://www.starjammersrd.com/equipment/starships/.
I don't see any notes on how much HP it gets in the book or online. Does anyone have a pointer?