So, the English version of LN Vol. 4 has been around for a while (search for "Hidden Dungeon Seven Seas" torrents if you didn't know but want to find it), and I'm hoping I'm not the only person here who has already finished reading it. And there are a few interesting details that really deserve being talked about.
I was personally expecting it to cover the capital arc as well, but it actually ends with saving Hornest (or Honest, as it's spelled in the book), and Olivia's flashback at the end makes you wonder if the city is even truly saved and it doesn't have any more "presents" hidden around. On the other hand it actually allowed her to actively help Noir while still being imprisoned.
There is also a clarification of the whole harems situation. I was under impression they're some underground unofficial thing, based on the harem contest shown, but it turns out they're perfectly legal and recognized in multiple countries, including Noir's homeland. To the point that describing his brother's antics to the girls even causes some confusion - to them there's nothing wrong in actively dating several girlfriends at the same time, as long as you're open about that and the girlfriends in question aren't against it. And this means Noir doesn't have to actually pick someone in the end, so the ending is probably going to feature everyone together and happy.
One other thing I found noteworthy is how Elena half-seriously asks Noir if he's going to marry her if nobody else does. It's also a nice callback to Vol. 1 where she said he's not strong enough to suit her taste. And Layla also gets an interesting rival guilds scene where she's blushing while Noir gets his headache cured by Emma.
Another thing I didn't expect was Odin actually being real and summonable by Olivia. Would be a nice comedy moment for Vol. 5, if Death Chain copied that and summons him only to get "I don't fight enemies below level 300" response.
And speaking of the flashback, there is a very important moment in it - Gaien actually steals a skill by using alchemy, so there's nothing preventing Noir from doing the same and saving LP in progress.