Loosing points after finishing 4th means that according to your MMR and your opponents’, you should have finished higher than that on average. To learn more about MMR:
As well as a recent change to how MMR works (if you are very highly rated, this would affect you and make it so that you loose more points on average):
I also wrote something to try to explain how ratings like MMR work, before the official post from Blizzard (first link above), you can read that too.