Am not entirely sure why "Rotten" is Season 4's lowest rated episode and rated lower than pretty much all the episodes of the previous three seasons as well. By 'Law and Order: Special Victims Unit' standards the rating is pretty low. Absolutely do not agree with it being the season's lowest rated episode and rated lower than a lot of the show's episodes. There are certainly far better episodes, but there are worse as well.
"Rotten" may not be a great episode but it is a long way from rotten. It is not as good as "Chameleon", "Dolls", "Damaged" and "Risk", the standout best episodes as far as the previous episodes go, but it is a lot better than "Disappearing Acts" (which is rated much higher somehow despite that its second half is a mess). The theme is not a new one and there is not much illuminating here, so maybe on that front "Rotten" may be a bit bland for some, but it is a topic that is always worth addressing with it still being true to life now.
If anybody finds "Rotten" bland at all, that is in a way understandable. As said, the subject of corruption within the force is not new territory and not much fresh is done with it, so it is on the predictable side. If one character was also written with more subtlety perhaps, it wouldn't have been as obvious that they were either involved in some way or knew more than what they let on.
Emotionally, "Rotten" doesn't quite have the tension, emotional impact or the shock factor of the previous two episodes for example. There is a twist, but while it is not easily foreseeable and is clever and well executed it isn't earth shattering.
Part of me cannot be too hard on the episode though because it does do a lot right. The production values are still slick and suitably gritty (without being too heavy in it). The music is not too melodramatic and is not used too much, even not being too manipulative in revelations. The script is taut, not too talky or flowery and makes one think. It has grit too and it doesn't feel trivialised.
From a story perspective, "Rotten" is not perfect. Yet it is still solid, not hard to follow and has enough intrigue even if other episodes of Season 4 and 'Special Victims Unit' in general have more to them on an emotional level. The team interaction is great and Olivia and Fin once again show that they gel very well as a team. The acting from the regulars is very good, as is that of Terry Serpico.
Summing up, far from rotten and quite good if not exceptional. Not the worst episode of Season 4 though, it's a lesser one but not the worst like the rating as of now implies. 7/10