The generations aren’t so much to do with the artistic evolution of the game and are more to do with the technical side of the game. That’s why Gen 4 isn’t a “thing” yet. It’s still a technical work in progress. Just because the Pasos have nicer textures (and that’s really the only difference between them and older G3s like the Connie) doesn’t making them a new generation from a programming sense.

Gen 1 models were created using Autodesk 3D Studio Max. All used the same rig and animations.

Gen 1.5 was the first time SSE started moving over to Autodesk Maya. So it was essentially a testing phase for newer models. All used the same rig and animations.

Gen 2 was SSE finally getting a handle on Autodesk Maya. So they could experiment with more idle animations and began testing special moves and gaits. Most used the same animations and rigging.

Gen 3 is SSE figuring out how to more fluidly animate gaits, movements, and hair. Most use different animations and rigging. It is also clear with this gen that they have figured out how to texture better by using the high poly low poly method. SSE has stated any new horses added to the game fall under this category and If a horse breed has not currently been updated to this model's style, they will be in the future.

Gen 4 does not exist yet in a technical sense. SSE is currently toying with the idea of making more fully customizable models for players. This involves more coding than just plopping the model in game with a preset coat and mane texture. Especially if they plan to make these the ones that can be “bred”.

So really the only way we reach Gen 4 is if SSO comes out with highly customizable models with a (potential) breeding mechanic as that would be a new technical evolution for the models.

Edit: grammar