Unavoidably, season 4 is the one that tends to be disliked the most as it did not involve the show frontrunner in any way. Had I not read that in many blog posts, I may not have noticed because in some ways the humor of the previous seasons is mostly there. Admittedly the laughs of other seasons are not there in anywhere near the same amount but this is not really the issue.
Instead of letting the relationships happen organically, there is a lot of group interaction/relationship stuff which is significant but yet also feels forced. This happens throughout and always feels like an 'idea' which was written towards, rather than part of everything else. This is made to work less well by virtue of it rarely being followed through into later episodes. Some elements do but mostly revelations, secrets, and conflicts do not get so much as a mention again and it makes it all feel a little hollow, as well as not as funny as it should be. The cast also feel a little restrained in their performances as the material doesn't offer them as much to work with, but it is Jeong and his character that suffers most as his plot device is annoying and ultimately pointless.
Too many episodes feel like uninspired retreads of better ones, and even with the many funny moments across the season, one is reminded of how much better it was in the first seasons. For sure it is not as bad as the internet would have you believe, but it certainly is not the best the show has to offer by some margin.