According to Damage Timing (p 34), effects that happen after a model was damaged comes to play only after the model was actually damaged.
As I can see, in case of Kentauroi, it will work thus. Damage flip is performed (2/3/4), damage is reduced by Abilities (e.g. Armor), remaining damage can be prevented by Soulstones. Then the model actually suffers final amount of damage. If it's more then 0, it's finally considered of being damaged at this point. Then +1 modifier comes to play.
The question is, what it can be applied to. I'd rather say, it can't modify already suffered damage from effect. But, according to FAQ, triggers are considered as additional effects of actions, so the modifier can be applied to them. According to Adding Damage section (p 24):
It refers Actions, but again, according to FAQ, triggers are also part of them. So I guess, in case of Kentauroi, the Rear Up will deal 3 damage to the target of an Attack and 2 to the others. Other two triggers will deal 1 damage (0+1).
Edit:
Have re-read my own answer. Want to add, that the wording "Models damaged by this Action" seems ambiguous. It can be interpreted as such that refers a fact that some models has been actually damaged (already accomplished fact). As well as modifying any damage that will be done in future.
Edit 2:
Have re-read my edit. The wording seems ok for me now. If it referred to accomplished fact, it would be something like "Models that have been damaged". So if it's about future damage, than only the effect itself and the Rear Up trigger can be modified.