I think part of the answer might be related to cost vs. amount of lift needed from the 2/4 helicopter blades. If you need a high lift helicopter, two blades ain't gonna cut it.
On turbo props though, I have heard that the six bladed props on the Dornier is alot quiter because of the extra blades. I think Jack is right about the point of deminishing returns on adding too many. High drag would only be a big problem if pitch was pushed up. Sorta like slamming on the brakes.