Seasons 1-3 are fantastic but Season 4 missed the mark.
The initial plot had potential but the development didn't make much sense and the deaths of characters - normally a big part of the show - were anti-climactic and boring. You really never knew what might happen next (not in an entertaining way).
Main characters died without any explanation (really, a random tornado??) and mostly only the least likeable characters lived. Finally, they made no attempt to connect this to the other series and it was set in mostly dry, dead Kansas with little snow and none of the great Fargo music.
A few quirky characters was really the only indicator that this was Fargo and not a random show.