"Anyone know the reason why McIntosh does not give this model a 4 Ohm rating? Especially if it supposedly tested as such?"
My best guess would be that McIntosh is known to be one of those brands you turn to if you need something very powerful, kind of like a Bryston or Krell. 100 McIntosh watts should be comparable to just about any other brand. But that's just a guess on my part. I agree with you in that they should have listed a 4 ohm rating. It can't hurt them.