Quick question as a non-native speaker:

A bestiary with 4 original monsters, 1 of which is unique (only one of them exists in the world) 

Does this mean there needs to be a 1d4 table so exactly one of the four is spawned per game? Or that I build four monsters, one of which only befitting for this flower? (Which would mean nobody has an imagination big enough to fit my creation into their own game?!)

I believe one unique monster is in reference to that being a monster which only appears once in your adventure/ hex.  As an example you could have four monsters: Bats, Hellhound, Skeleton Knight and Grave-Dragon. There is only one Grave-Dragon but there are multiple hellhounds, bats and skeleton knights roaming your hexes. That is how I interpreted this requirement.

Original monsters means monsters that are new and written by you, so not classic enemies like a wolf, bear, skeleton, or dragon. These are something you wrote that can't be found in other lists of monsters/bestiaries. The unique monster is a single creature, there will only ever be one.

So you should have three monsters you wrote that are more of a species and you might encounter multiple different individuals (like specimens of the same species) or a group of them and one monster that has no repeats, if you encounter it multiple times it's the same individual. For example, the party might run into dozens of blood moths (one of the 3 original monsters), but only one Moth Witch (the unique) and once she's defeated they won't meet her again.

(Also, you can include classic/non-original monsters in your hexes, they just don't need a bestiary entry. So if you want roaming wolves on your map or as an encounter put them in. Whoever grabs your map can use whatever set of 'wolf' stats they want.)