The Changbai Mountains did not reach an altitude of 4800 meters. The highest peak of the Changbai Mountains was the White Cloud Peak, which was 2691 meters above sea level. The highest peak in North Korea was the General Peak, which was 2749 meters above sea level. Therefore, the Changbai Mountain was not more than 3000 meters above sea level.
Well, to determine if 'Above the Cloud Sea' light novel is popular, we need to consider different aspects. If it has been translated into multiple languages, has a large number of reviews on relevant platforms, and has an active fan community, then it could be considered popular. However, I haven't specifically looked into this light novel, so I can't say for sure. There are so many light novels out there, and some hidden gems might not have received as much mainstream attention as they deserve.