Season 1 through 4 are rather irrelevant in comparison to today’s matchmaking. There have been too many changes since. Check Kawumbaa’s guide for details:
The rating and matchmaking system is confusing, and a good overview does not appear to be available. The official overview (1, 48) is incomplete and does not answer a number of common player questions and concerns. This information below is gathered from sporadic blue posts and developer update videos, and salted with my own experience and experiments, various forum threads, and watching streams. Note that since Blizzard does not give exact algorithms, I do have to fill in some gaps, or leave so…