In a nutshell, volume 4 of Goblin Slayer is enjoyable. It's nothing extraordinary, but certainly not bad, either. It starts with a look into how promotions work in the Adventurer's Guild, and introduces a character that will most likely show back up at some point, so it's not just a throwaway side story. The bulk of the volume focuses on a quest the party receives to exterminate the goblins who are living in the sewers under a large city. And I have to hand it to the author for making the "sewer mission" trope actually interesting. The combat throughout this section is fun, and well-drawn, and we get to see just how cunning the goblins can be with traps, and adaptations to their environment. There are hints of something bigger at work behind the scenes, but what it is hasn't been revealed yet.
Volume 4 does end on something of a cliffhanger, which is nice. Because, while the fight scenes are nicely illustrated and engaging, and the foreshadowing of a nebulous doom lurking in the (perhaps not so distant) future are nice and all, this volume doesn't really do much that we haven't seen before, when you get down to it. Ending the book in the middle of a quest provides a little extra incentive to keep reading, if the wholesale slaughter of goblins is starting to lose its luster. Of course, volume 5 is probably going to pick right up with more of the same, but at least this is a longer quest that has the potential to throw some more surprises at us.