"Why do I spend time thinking about this stuff?" -- cb
First off, I agree with what you figured out about the meanings of "hm", "hmm" and "hmmm" and hence I'm happy my fondness for that word no longer coaxes me to contribute my views. The importance of this topic I think lies in the interjectional character of that sound: As it has very little of this tangible semantic kind of meaning, the word has great value for pointing out how important this other layer of meaning can be. Perhaps it could be called something like "connotative", but that word I think is not as apt as I would like it to be.
I agree that four is excessive in most cases. I think two is right for thinking, as in: "Hmm, I think the answer is 42." But three is for questioning, like when someone says something to you and you're only half listening. So it really should be Hmmm?
I am usually pretty careful about how many M's I use with my Hm or Hmm. But generally two is enough for almost any purpose. "Hm" is more like a "hmph" without the peevishness, and "hmmm" implies a level of thinking that is not serious enough to be remarked upon.