How similar are the stories for the four characters?

It's rather obvious that they'll be at least some differences between them. Katarina's a rather big part of the main quest near the beginning with Lucas, but if I were to play it over again, I'm wondering if it's still mostly the same or if it's like an entirely different campaign, and whether or not the questlines merge together sometime.

There are certain points where you interact with NPCs, and the dialogue with them plays out the same regardless of who you're playing, with some minor variations.

For example:

When you first meet Marten Guiscard in the burning mansion, both Katarina and Anjali have the same dialogue choices.

But:

If you play as Katarina, when you first run into Odo, you can ask about his ward Anjali as well as Lucas. If you play as Anjali, you obviously don't get the option to ask about yourself.

When you come to a point where you'd normally interact with a main character but can't because you are the main character you're supposed to be interacting with, the game uses a different (essentially generic) NPC instead. For example,

When playing as Katarina, you get propositioned in Raven's Rill by a guy named Alexei instead. It's still exactly the same quest.