Frankly, I wasn't expecting this to live up to Season 3. That season was attacking the root of the entire dungeon-bashing genre. Our heroes' day-to-day job involves killing monsters. What would that look like from the monsters' viewpoint?
Season 4, on the other hand, is just dungeon-bashing. Our heroes spend 22 episodes fighting monsters underground. Simple as that. (Well, more or less. Our heroes get a break between the season's two story arcs in eps.1-5 and eps.6-22.) There's some dark material about Ryuu's past and the loss of her familia, but nothing particularly surprising. Her head's messed up. Yeah, we got that. She has survivor guilt and thinks she's a criminal for her revenge murder spree against her familia's killers. (Whose side are we on? Well, let's just say that her enemies' name was "Evilus".) There's nothing wrong with that material and you could tell strong stories with it, but it's not the slam-dunk that Season 3 was.
That's why I'm almost more impressed by Season 4 than I was by Season 3. More heavy lifting was needed to become this powerful.
The introductory story (eps.1-5) is quite good, but it's just fighting a monster. Nothing too remarkable there. You will, though, believe that this monster is going to single-handedly kill the entire party, despite their combined firepower. Bell gets a power-up and meets a topless mermaid.
After that, it's discovered that Ryuu's been mutilating and killing people. A bounty is placed on her head and a party is dispatched to hunt her down. This makes our heroes uncomfortable because they owe her their lives, but what if the stories are true? They join the bounty hunters. The resolution of this puzzle involves an over-the-top villain who, I think, weakens his episodes. He's a massive ham. He can't stop shrieking and gloating. His scenes would have been stronger if he'd had some reality instead of being a cartoon, but it's what comes next that really matters. Bell and Ryuu fall to the lower levels. They're going to have to deal with Ryuu's inner demons while at the same time fighting off monsters that could eat alive an army.
Basically, they spend 11 episodes on the brink of death at any moment. The only cannon fodder is Bell and Ryuu. Dive that deep and there's no such thing as a weak monster. They're all lethal and there's no way out. It's like a war movie, with our heroes stuck behind enemy lines and just waiting to catch a bullet. This is strong. Sacrifices become far more meaningful. Even the simplest scene has more force. It's capable of being reminiscent of Made in Abyss, e.g. ep.17 and drinking an ooze.
The episode that particularly impressed me, oddly, was Welf Crozzo making a sword in ep.16. There's nothing special about the story idea. Blacksmith makes sword. Well, d'oh. That's his job. He's doing it under extreme circumstances, yes, but even so you wouldn't expect "I made the sword!" to be a dramatic high point.
But it is. It's stirring. I was astonished. It takes real skill to craft an episode that powerful out of story elements that are, theoretically, a bit dull.
Similarly, we regularly see flashbacks to Ryuu's days in the Astria Familia, before everyone died. We see a new side of Ryuu and the scenes are pretty good, but I wasn't expecting to be as horrified as I was on eventually seeing the familia's death in ep.20. Bloody hell. Again, the writing's more powerful than you'd expect from the material... and we'd already known that that day was earth-shattering for Ryuu.
I'm in two minds about the subtitles I saw. They're surprisingly free translations, e.g. lots of swearing. I don't disagree that that's one way to punch up the impact in English, but the subtitles also wreck that ep.16 "seigi" discussion by translating it as "justice". Yes, that's a legitimate meaning of "seigi", but it's a narrower one than the original Japanese word and it doesn't fit some of the discussion points at all.
I'd also have been happier if this show's world didn't run on game mechanics. That's normal these days in anime, but even so. This is a dungeon where monsters spawn and the rooms regenerate.
This season's really, really good. It might not seem like much in the first half, but stick with it. It's a bit drab, serious and po-faced (although it can make me laugh when it wants), but for me it's the definitive sword-and-sorcery fantasy anime of the past decade. Yes, above Sword Art Online. It's unrecognisable from its early, more fanservice-heavy days. After this, I'll watch pretty much anything that comes out of this franchise.