A four point drop? It is hard for me to imagine this as a penalty, per se, because I can't quite see it having much effect on the webmasters being penalized. Yes, they might panic, review their site, make changes, but in the end, they would most likely write it off as an algorithmic issue. I doubt it would have the corrective effect that knocking the offending site off the front page would.

My suspicion is that most likely this would be a result of changes in how some factor affects ranking. Possibly some type of link or link scheme is being detected and filtered out, or changes have been made to the way design elements are filtered out, which affects the way Google determines how relevant these pages are to the query. Personally, I would look for commonalities between the affected sites. If the sites were designed or optimized by the same company, I would look at common factors on page. If the sites share an advertising firm, or have partner sites or major advertisers in common, I would look off page.

But really, something like this change can become very difficult to track down, assuming that this is an algo change rather than an actual penalty, since there is no way to know how close a match the results are or were. For example, results 1 and 2 for a certain term could be 99.989% and 99.971% matches, and 1 and 2 for a certain other term could be 99.999% and 99.997%. If both sites in the number one position share a designer, who did something wrong that Google now filters for, they might each lose a hundredth of a %, so they become 99.979% and 99.971% for term one, 99.989% and 99.997 for term two. This means that there is no change in the SERPs for term one, but for term two, the sites would actually switch position. So you have no way of figuring out that it is actually that one commonality that only caused a loss of rank to the second #1 site.

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