In the 1983 Polish Cup final, Lechia Gdansk (of the 3rd level) beat Piast Gliwice (2nd level). Given that the 2nd level in Poland consisted of two parallel divisions (A and B), this means the cup was essentially won by a fourth division team.
Along the way, Lechia beat:
Ruch Chorzow [1st level, finished 3rd in league]
Zaglebie Sosnowiec [1st level, 11th]
Slask Wroclaw [1st level, 6th]
Widzew Lodz [1st level, 2nd]
Olimpia Elblag [2nd level]
The runners-up Piast beat
Lech Poznan [1st level, 1st]
Wisla Krakow [1st level, 5th]
Baltyk Gdynia [1st level, 10th]
Gwardia Warszawa [1st level, 16th]
Lechia followed their Cup win by also winning the Supercup, beating league champion Lech Poznan.
Sadly Lechia couldn't take their giant-killing form into Europe, being battered 10-2 on aggregate by Juventus in the first round of the ECWC, but those matches are historic for reasons beyond football, due to the political anti-government support of the crowd in Gdansk.