Hibees made light of the imminent departure of star striker Gary O'Connor to Lokomotiv Moscow with three goals before the break.
Top scorer Riordan opened the scoring with a low 20-yard effort after only six minutes.
And things got worse for United after 25 minutes when Riordan floated a cross towards Chris Killen at the back post. Alan Archibald attempted to intercept but only succeeded in bulleting a header into the roof of his own net.
But the pick of Hibees goals came when Killen ran on to a peach of a pass from Riordan, twisted away from Garry Kenneth and slotted home a lovely finish.
Hibernian keeper Simon Brown made his first save from a Barry Robson drive, but it was all Hibs as Craig Samson produced fine saves from a Paul Dalglish drive and a close-range effort from Ivan Sproule.
It was so bad for United that Collin Samuel managed to mis-hit his shot straight at Brown from 10 yards, but young substitute David Goodwillie did manage a consolation goal in the last minute after Samue'ls cross came back off the bar.
However, it looks as thoug United boss Craig Brewster has an uphill battle in front of him.
A