Lech's Swedish striker Mikael Ishak opened the scoring in the 29th minute in Denmark as the visitors launched their European campaign in dominant fashion on Tuesday.
Swedish winger Patrik Walemark added two goals, while Ghanaian defender Terry Yegbe also found the net.
Kristian Arnstad netted the lone goal for the hosts, who were reduced to 10 men in the 37th minute after Jesper Hansen received a red card for a handball, Polish state news agency PAP reported.
Kristian Arnstad netted the lone goal for the hosts, who were reduced to 10 men in the 37th minute after Jesper Hansen was sent off for handling the ball, Polish state news agency PAP reported.
The return leg will be played in Poznań next Wednesday, July 29.
The winner of the two-legged tie will face either Sabah of Azerbaijan or KuPS Kuopio of Finland in the third qualifying round.
To reach the Champions League group stage, Lech must defeat Aarhus, the Sabah/KuPS Kuopio winner, and one additional opponent.
Meanwhile, Polish Cup winners Górnik Zabrze lost 0-1 away to Turkish giants Fenerbahçe on Tuesday in the first leg of their Champions League second qualifying round tie. The return leg will be played next Wednesday in Zabrze.
Lech won the Polish premier league championship in May for the 10th time, finishing four points ahead of Górnik, who finished second.
(gs)
Source: TVP, IAR, PAP