I have bought volumes and read it. It’s good. It has just 4 volumes in total.

Can you please provide a descriptive summary about that happens next ? :)

Where can you buy the fourth volume? I have been looking everywhere but I can’t find it.

Maybe he meant 3 since it only has 3 volumes in total and it's already completed in Japan. The three volumes are already published in English by VIZ. You can check the manga information here https://www.mangaupdates.com/series.html?id=139385