I admittedly don't know about IV because I haven't played it, but I-III were light when it comes to the story, not much more than the average game in the genre. I think you might be getting fooled by the artstyle or its publisher/developer; the story is really just there to give a reason for the dungeon and its enemies inside to exist. It's all about the gameplay.
Of recently-released first-person dungeon crawlers, Unchained Blades had the most story to it. That particular game has so much story to it that I actually wanted it to be quiet and let me just go through my dungeons.
But, absolutely, try the demo. You might find you enjoy the genre more than you'd expect.