Well, it's mainly about a girl's journey in a marriage. She might face various challenges and also experience some heart - warming moments. But to know the exact plot, one really needs to read the novel.
It could be the way the story is told. The author may use a special narrative style in 'My Happy Marriage Vol 4'. The details of the setting, the dialogue between the characters, and how the emotions are conveyed can all contribute to making it a special addition to the series. Maybe it dives deeper into the cultural context that gives the story a unique flavor.
Well, perhaps more character development. In previous volumes, we saw the relationship between the main characters evolving, and in volume 5, it might continue that trend. There could be new challenges for the couple to overcome, which would further strengthen their bond.
In terms of plot, it might be more convoluted. The previous volumes have laid the foundation, and this one could build on that with more intricate storylines. Maybe there are more sub - plots that intertwine with the main plot, making it a more engaging read.
There is 'The Saint's Magic Power is Omnipotent'. Although not directly about marriage, it has elements of a relationship developing in a fantasy world which might be of interest if you liked 'borrow my happy marriage vol 1 light novel'.