Fargo Season 4 won't start shooting until fall 2019, so it likely won't return until 2020

Entertainment Weekly

“We have a year, and we have a city (that season 4 takes place in),” says executive producer Warren Littlefield. “The plan would be to go into production and start shooting in the fall of 2019. Noah (Hawley) has a lot on his plate. Noah knows when it is and where it is, and he’ll commence writing that after he shoots his feature with Fox Searchlight this summer. So in the fall, he’ll begin writing and with Fargo we generally have most of the scripts written before we go into production.”