>>101191098S1 took place sometime in 2014
S2 sometime in 2015
S3 mostly also takes place in 2015, but the last few episodes are in early 2016 (Bojack and Sarah Lynn's binge in S3E11 is specifically dated as starting on January 15, which was in both the show and the real world the day after the 2016 Oscar nominations were announced; if the time between the announcement and ceremony is the same in the show's world as in the real one, that means the binge lasted until February 28, a total of 44 days)
S4 starts pretty much where S3 ends, though it fast-forwards through most of the year and ends in early 2017.
