In 2017 I was able to visit:
January 31-February 2: This was mid-week and had the least amount of crowds I have ever seen at WDW.
April 8-15: (Week before Easter) It was slightly busy, but it was very enjoyable. Popular rides had lines, but we didn't wait more than 20 min or so for most of everything.
May 27-June 4: Crowds on the weekends were a little larger. This was the week Pandora opened and we were there opening day. As expected Pandora was real busy (they began a line system to allow so many people in at a time). We didn't have any issues with crowds at all. Wait times were 20ish minutes.
August 4-9: A little more crowded than the May/June trip, but again, I didn't find it unbearable. Wait times could get up to 30 minutes for rides.
November 18-24: (Thanksgiving Week) Crowds were not large until Tuesday when we noticed them increasing significantly. Wait times were definitely longer at the parks we visited. I never felt that they were over the top crazy, but I have been there enough to not care as much to see the busy attractions.
In all, I wouldn't think of your 4 trips as 4 individual trips, but as 4 parts of one vacation experience. That should be enough that you can hit the big attractions with your fastpasses and explore the rest. Get an idea of all the things you definitely want to see and break them up. I think it is more enjoyable to us when we do it that way.