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.
Episodes three and four are always a better indication of where things are going and that is why I usually give any series that same window to impress or lose me, thus applying the 'Four Episode Concept' as a method.
That said, this is by no means an absolute method. Three or four episodes are a good enough amount for most 16-18 episode dramas, but there are exceptions and longer dramas also take longer to settle into their routine. Furthermore, if a series simply repulses you or if you adore it during the first two episodes, it's unlikely things will change that radically in the next two. The big changes usually come with sudden problems or then during the final stretch; an unfortunate time for any series and a lot of viewers. But that is a whole other topic.
They had me at "Is that your lame excuse, you *******?!"
Be it four episodes, six or more, a drama will only really pick up after the first, introductory act. No matter how pleasant a work is, it needs to have a good and solid beginning, middle and end to be really good. We often watch dramas just to pass the time and this approach does not mean much when we don't invest valuable time and emotions, but if your time is limited and you are looking for more than just an average show, this is a good method to go by.
Of course, if time and mental capacity are limited, it's also a good idea to wait until the series is over and have a better overall view of it from reviews and opinions, but if watching it as it airs and while the buzz is ongoing entertains you better, the Four Episode method can also help you find out what is worth watching now and what might be a good idea to leave for later.