First one that crossed my mind was Bonecrusher Smith in his pro debut getting KO'd in the 4th round by 2-0 James Broad, but after double checking on Boxrec, that one was scheduled for 6. Maybe Terry Norris' first fight with Gilbert Baptist? That was a UD4 for Norris. They fought a second time, a UD10 for Norris, and I believe it actually was the main event.
Surprised no one yet mentioned Roberto Duran w4 Carlos Mendoza! Mendoza went on to fight Wilfredo Gomez for a world title and Duran..well....Duran.......went on to the galaxy.
John Michael Johnson lost relatively early in his career to both Junior Jones (LUD4) and Johnny Tapia (LUD8) in his 10th and 11th bouts respectively. He eventually avenged the loss to Jones by winning a world title fight against him (TKO11) in what was Johnson's 22nd pro bout.
Like you, I thought I had one with Curtis Cokes beating Manny Gonzalez in his pro debut, but again, it was scheduled for 6. Tough question.
Riddick Bowe's pro debut came in a four-rounder against Lionel Butler, who later knocked out Tony Tubbs, Bonecrusher Smith and fought Lennox Lewis for the WBC mandatory slot.