If we have to define it, it's an extra part spin out of canon story. It's a unique leaf of a melody that Urobuchi Gen conducted from a completely Fate-ish trunk. Here, you don't have to consider the content of the Fate/stay night game. Only move the story according to your wishes, let the characters shown portray themselves to the fullest and rush towards the ending without regret.
Gen: Talk about it, what is the fourth war about??
Nasu: Hum ? Saber getting bullied by Gilgamesh and Alexander!
In my mind, there are two main points in the fourth war.
One is Saber's setback.
King Arther, who ruled her people as the model human. The selfless and devoted, just and uncorrupted Arturia.
The absolute commander who surpassed mortals, the majestic Gilgamesh.
And the extravert king whom some called a tyrant, but still held his belief that he'll bring his people to happiness. The king of conquerors, Alexander, who ruled the world as a man.
Using the intensive conflict between the three to unravel Saber's story in Zero.
And the other point is Kiritsugu's story. I contracted that to him according to his wishes as well. I don't care about other things anymore.
"I understand. Then I'll do it according to the way I like it to be. Ah, but I'll do my best to get the style of the piece close to Fate."
I think Urobuchi Gen worked a lot more on it.