Pathfinder has made a great number of wonderful changes from D&D 3.5, but there is one change that I am having a difficult time understanding. In the 3.5 Monster Manual, on page 290, it says that "whenever a monster reaches a number of Hit Dice divisible by 4, it improves one ability score by 1 point." This would imply that the monster's racial Hit Dice is equivalent to character level, i.e. a character gets an ability increase every 4 levels, while a monster gets an ability increase every 4 Hit Dice. In the Pathfinder Bestiary, on page 296, it says that "for every 4 additional Hit Dice gained by the monster, add 1 to one of its ability scores." The word "additional" here seems to imply that the creature gets its first ability score increase four levels AFTER whatever Hit Dice it has at base in the Bestiary. This implies that Hit Dice are not equivalent to character level after all, since a character now gets an ability increase every four levels while a monster gets their first ability increase at four Hit Dice PLUS whatever racial Hit Dice they already have. This seems odd, since the Bestiary also specifically says that racial Hit Dice give character feats as normal (1st level, and every two thereafter). Is this a change from 3.5 or am I simply misunderstanding? If it is a change, why the change? I checked the errata, but it didn't say anything about this. DESIGNER COMMENT PLEASE! THANK YOU!! =)
TLDR: Are racial Hit Dice equal to character levels for ability score increases? Do monsters gain an ability score every four Hit Dice just as characters gain one every four levels? If not, why the change from 3.5?
I think you can safely ignore the word 'additional' in that sentence. I think most, if not all, of the monsters follow the 'every 4 HD' approach, so just go with that when you advance them. This goes both both ability score increases, feats, and anything else based on HD or level.