Season 4 Episode 1 of Teachers has some of the characters visiting the graves of three characters from the previous three seasons:
Kurt, Brian, and Matt.
In the synopsis on IMDB it says they died in a car crash. I didn't hear anything during the episode that specifically said it was a car crash that killed them. Is there some other source the IMDB synopsis got its information from? Was there perhaps a "Christmas Special" episode or similar, something shown between the seasons but not available on Netflix for some reason?