The Asian champions, who won with goals by Mauricio Molina, Sasa Ognenovski, Choi Sung-kuk and Cho Dong-geon, will meet the Italians at the same Zayed Sports City stadium on Wednesday.
Seongnam coach Shin Tae-yong said reports that Inter coach Rafael Benitez's job hinged on winning the tournament was a sign there was trouble in the opposition camp.
"I'm sure there is a weakness because of the conflicts at home," Shin told the post-match news conference.
"But Inter are the top team in the world. It won't be an easy game but our players are here to do their very best. I'm sure they will give a great performance against Inter."
Colombian midfielder Molina gave Seongnam an early lead against al-Wahda with a left-foot volley from the edge of the area after a poor clearance by defender Hamdan al Kamali.
Brazilian striker Fernando Baiano equalised with his second goal of the tournament in the 27th minute when he headed the ball home from right-back Eisa Ahmed's cross.
However, the South Koreans were in front again within three minutes when al-Wahda defended poorly at Molina's outswinging corner from the left and Seongnam's Australian captain Ognenovski scored with a free header.
Choi made the win safe in the 71st with a shot from the right that gave goalkeeper Adel al-Hosani no chance at the end of a move started by Molina.
Fellow forward Cho headed their fourth off Molina's curling free-kick nine minutes from time.
Shin said he considered striker Samuel Eto'o as Inter's dangerman but his team would try to make the most of their unique chance against the Champions League winners.
"There might never be another opportunity like this so we will make the most of it," he said.
"There is a Korean proverb that says 'better than dead'. We will challenge ourselves and show them we can be in same game as them."
