The fact that a reviewer uses 2 and 4 ratings might not be too meanngful. One reviewer might use 2s and 4s the way another uses 1s and 5s.
There may be reviewers who do not generally fit the bimodal distribution, but if they are the minority, why design your whole rating system around them? Better to optimize your system for the way most of your users behave.
There may be reviewers who do not generally fit the bimodal distribution, but if they are the minority, why design your whole rating system around them? Better to optimize your system for the way most of your users behave.