Adding class levels to a monster with only 1 HD; is the Hit Die replaced?

[Faolán Maiali the Azure Abjurer]

So I'm looking over the Brinebrood Queen in the Skull & Shackles AP, and I noticed that she's a grindylow aquatic druid 4, yet she has 5 Hit Dice. I'm confused, because grindylows only have 1 Hit Die. Don't non-classed creatures with only 1 Hit Die in the Bestiary have that Hit Die replaced if they take a class level? I know that's how it worked in 3.5e for all creature types, as per the sidebar in Savage Species:

"A creature with a single Hit Die, like all standard-race characters, gives up that Hit Die when it gains its first class level."

[Java Man]

Humanoids with 1 racial hd do exchange it for class levels, but grindylow are aberrations.

[BENSLAYER]

Java Man wrote:

Humanoids with 1 racial hd do exchange it for class levels, but grindylow are aberrations.

Do you have a link for this? I cannot find a reference for any Creatures exchanging 1 Racial H.D. but that could just be me missing it. If so, I might have a couple of stored N.P.C.s to adjust ...

N.B.: I could only find this relating to the first H.D. being Racial, and this which refers to Racial H.D. Stacking with Class levels, (in that particular scenario). Neither mention replacement by Class Level.

[Quixote]

Unless something changed from 3rd, that first HD is usually a class level, like warrior. See: goblin, orc, hobgoblin, etc. So a lvl4 druid doesn't have 5HD, because it's not also a lvl1 warrior. But if it's an actual racial HD, class levels don't replace it.

[Java Man]

Rules for humanoid type, monster index on AoN:

"Humanoids with 1 Hit Die exchange the features of their humanoid Hit Die for the class features of a PC or NPC class. Humanoids of this sort are typically presented as 1st-level warriors, which means they have average combat ability and poor saving throws. Humanoids with more than 1 Hit Die are the only humanoids who make use of the features of the humanoid type. A humanoid has the following features (unless otherwise noted in a creature’s entry)."

[GotAFarmYet?]

Faolán Maiali the Azure Abjurer wrote:

So I'm looking over the Brinebrood Queen in the Skull & Shackles AP, and I noticed that she's a grindylow aquatic druid 4, yet she has 5 Hit Dice. I'm confused, because grindylows only have 1 Hit Die. Don't non-classed creatures with only 1 Hit Die in the Bestiary have that Hit Die replaced if they take a class level? I know that's how it worked in 3.5e for all creature types, as per the sidebar in Savage Species:

"A creature with a single Hit Die, like all standard-race characters, gives up that Hit Die when it gains its first class level."

You are confusing a few things

CR value is more aligned with Class Level
HD is assessment of HP 1 HD was suppose to be a 1d8
Druid 4th level is just to help you understand the power the monster has learned by living.

so in NPC terms think of it as a 4th level druid with a bonus role for a possible High CON score.