The 'Four Episode Concept' is simple. It basically means that, when it comes to Korean drama, you can't know what the overall feel and quality of the series will be after one or two episodes. There is a good reason for that. It's the way Korean dramas are made and broadcast. Most of them have two episodes per week, each lasting one hour. This means that, in order to get ratings, the ultimate goal of dramas, viewers have to get interested in them during those two episodes.
In order to get viewers interested, the production team will attempt to pack the first two episodes with things that will get people to sit and watch, not necessarily things that are good or serve the story. As a result, we end up with many Kdramas having very different pilot episodes to what the rest of the series will be like. Sometimes the start is good and things deteriorate, sometimes we get mass-friendly starts that turn into great shows.
Tell me again how our first two episodes fooled you into watching.