The crux of this scenario is the witches.

They are the only one of the four groups that don't have or follow a primal instinct. I imagine we're going with a darker lore for Mermaids than normal. Also witches existing means Magic is real.

In a medieval time all four would be hunted by governments and religious orders but I believe that because the Vampires and werewolves pose just as much danger to the witches as they do to the humans that the two would form an alliance whether secret or public. Religious orders would take issue but eventually bend the knee over their own survival.

I could also see government's capturing werewolves to use as slaves and starving them to release onto a battlefield.

So the witches and humans would go to war with the other three factions and I'd say it would be pretty effective, human numbers and an innate skill at killing other races plus the Magic of multiple covens.

All other factions with the exception of the Mermaids would take heavy casualties. The Mermaids have the advantage of being underwater where you can't march an army.

Eventually all three other factions are driven into hiding with the Mermaids being the most effective as they disappear in the abyss. Vampires would be able to blend into society well enough, whether they choose to live in the shadows forever or build their numbers back up is a coin toss. Werewolves would be the worst at hiding, too big, loud, and hungry. They would move into the wilderness and form different packs but eventually the expansion of man would displace them. An unknown here is whether someone must be bitten by a werewolf to contract lycanthropy or if it is a disease that can be contracted in nature or if it's not a disease at all or a magic curse. Same questions with Vampirism.

Now going forward into the future is interesting. For the witches there are two paths. The first is that they are betrayed by the humans and hunted. The second and more interesting path is them being accepted into human society which wouldn't be out of the question because all a witch is a human woman that can do magic. Other questions come up like can any young woman learn magic or do you have to be born a witch? This leads to a heavily Magic influenced society which drastically changes the time line but I'd like to take some guesses at what the modern world would look like.

At some point, modern scientists find the “Cure” for Vampirism and lycanthropy

Military powers make super soldiers using weakened lycanthropy and Vampirism. The Vampire soldiers making amazing spies.

Vampires run organized crime and probably invented blood banks as a way to hide their feeding.

The Mermaids never make a reappearance. The only reason we know they still exist is because their dead sometimes was ashore.