Well at home i have only 4Mbit too but it works great with HD-Streaming (720p).
I would like to emphasize the word "household" in my previous post. Statistically, 2.5 people live in the average US household, which share the bandwidth I mentioned.
As soon as multiple people are involved, 4Mbit is rather limited (for video streaming).
Also, "works great" is obviously subjective. My 16MBit internet connection is relatively fast but 720P video streams do load annoyingly slow quite often (YouTube, etc.). Most viewers expect near-instant video streaming based on their TV viewing habits. If BluRay is to be replaced, buffering data for multiple minutes will have to be a thing of the past.